目錄
In Korean, 「오빠、언니、형、누나」 may all look like words for “older brother” or “older sister,” but when you actually use them, the real difference lies in who is addressing whom. A woman calls an older man 「오빠」 and an older woman 「언니」; when the speaker is a man, those same two people become 「형」 and 「누나」. And just because someone is older does not mean it is automatically appropriate to use one of these terms the first time you meet. How well you know each other and what kind of relationship you normally have also affect which form of address sounds natural.
「선생님」 is a different case. The word can refer to a teacher, but it is also used in situations where you do not know someone’s name or job title and need to address them politely. So translating it simply as “teacher” covers only part of its actual usage. What makes Korean forms of address genuinely confusing is that these words simultaneously encode age, relationship, and degree of politeness. If you memorize only their Chinese equivalents, it is easy to misuse them in real conversations.
How Do You Tell 「오빠、언니、형、누나、선생님」 Apart? It Depends on Who Is Addressing Whom
Korean has four different forms of address corresponding broadly to “older brother” and “older sister,” and the distinction depends not only on the person being addressed but also on the speaker. This is one of the layers that Chinese most easily leaves out. Simply memorizing “오빠 = older brother” and “언니 = older sister” is not enough, because when the speaker is male, the same older man becomes 「형」 and the same older woman becomes 「누나」.
| Speaker | Other Person | Common Form of Address | Related Honorific Form | Can It Be Used for Non-Family Members? |
| Female | Older male | 「오빠」 | 「오라버니」 | Yes, but usually only with some degree of familiarity |
| Female | Older female | 「언니」 | No separate standard honorific form | Yes, commonly used between women who are close |
| Male | Older male | 「형」 | 「형님」 | Yes, commonly used between men who are close |
| Male | Older female | 「누나」 | 「누님」 | Yes, but it usually implies familiarity |
| Any | Teacher or someone who should be addressed politely | 「선생님」 | Already contains the honorific 「-님」 | Yes, and it is relatively formal |
This table first answers the question of “who can call whom what,” but in actual use there is another layer to consider: whether the relationship is close enough. Someone being older than you does not mean you immediately call them 「오빠」 or 「언니」 the first time you meet. These words can reduce interpersonal distance, and precisely because of that, using them too early with someone you barely know can sound as though you are moving into a more personal relationship too quickly.
How Is 「오빠」 Used? A Woman Addressing an Older Man—It Does Not Mean “Boyfriend”
「오빠」 is used by a woman to address a man older than herself. The most basic cases are an older biological brother or an older male cousin, but it can also be extended to older men she knows well. It often appears in conversations between couples in Korean dramas because a woman may already call an older boyfriend 「오빠」. That does notmean that 「오빠」 itself means “boyfriend.”
- Example: 오빠, 오늘 시간 돼요?
Romanization: o-ppa, o-neul si-gan dwae-yo?
Translation: Oppa, are you free today?
This is natural when speaking to an older man you already know well and with whom this form of address is mutually accepted. It sounds familiar and close. - Example: 친오빠가 한 명 있어요.
Romanization: chin-o-ppa-ga han myeong i-sseo-yo.
Translation: I have one biological older brother.
「친오빠」 specifically means a biological older brother and is different from the broader use of 「오빠」 for an older man you know. - Example: 오빠라고 불러도 돼요?
Romanization: o-ppa-ra-go bul-leo-do dwae-yo?
Translation: Can I call you 「오빠」?
If the relationship is not yet close enough for you to decide on the form of address yourself, asking first is more natural.
The most common misunderstanding about 「오빠」 is treating it as a general affectionate name for men. In reality, a man normally does not call an older man 「오빠」, and a woman who does not know an older man does not need to call him that simply because he is older. The form of address itself carries information about the relationship; it is not merely an age label.
How Is 「언니」 Used? A Woman Addressing an Older Woman, Including Outside the Family
「언니」 is how a woman addresses an older woman. It can refer to a biological older sister or an older female cousin, but it can also be used for a close senior, friend, or female acquaintance. Compared with 「오빠」, which is often romanticized in dramas and other media, extending 「언니」 to non-family members among women is actually extremely common in everyday life.
- Example: 언니, 이거 어디에서 샀어요?
Romanization: eon-ni, i-geo eo-di-e-seo sa-sseo-yo?
Translation: Unni, where did you buy this?
This is appropriate with an older woman you are close to and is common among friends, seniors and juniors, or acquaintances who know each other well. - Example: 친언니랑 같이 살아요.
Romanization: chin-eon-ni-rang ga-chi sa-ra-yo.
Translation: I live with my biological older sister.
「친언니」 clearly indicates a biological older sister. If you say only 「언니」, it may also refer to a familiar older woman depending on context. - Example: 언니라고 해도 돼요?
Romanization: eon-ni-ra-go hae-do dwae-yo?
Translation: Can I call you 「언니」?
When shifting for the first time from a polite relationship to a more familiar form of address, confirming whether the other person is comfortable with it is safer.
You may also occasionally hear 「언니」 in restaurants, hair salons, or shops, but this usage depends on the speaker’s age, the culture of the establishment, and the degree of familiarity. It should not be treated as a fixed term for all female staff members. If you are unsure, using a job title, a name plus 「님」, or a more neutral polite form of address is safer.
How Is 「형」 Used? How Men Address Older Men
「형」 is the word a man uses to address an older man. Its basic use corresponds to a biological older brother, but it can also be used for a close senior, friend, or acquaintance. If a woman were addressing the same older man, she would normally use 「오빠」 instead, so the main difference between 「형」 and 「오빠」 is not the person being addressed but the gender of the speaker.
- Example: 형, 같이 밥 먹어요.
Romanization: hyeong, ga-chi bap meo-geo-yo.
Translation: Hyung, let’s eat together.
This is suitable when a man speaks to an older man he knows well, and the term itself already conveys familiarity. - Example: 아는 형이 서울에 살아요.
Romanization: a-neun hyeong-i seo-ul-e sa-ra-yo.
Translation: An older guy I know lives in Seoul.
「아는 형」 commonly refers to an older man who is not a relative but with whom the speaker has some degree of personal connection. - Example: 형이라고 불러도 될까요?
Romanization: hyeong-i-ra-go bul-leo-do doel-kka-yo?
Translation: May I call you 「형」?
This is a more polite way to confirm a form of address and is appropriate when two people are just beginning to become closer.
There is also a more respectful form, 「형님」. It can be used as an honorific for an older brother, but it also appears in relationships between men where hierarchy or etiquette is emphasized. 「형」 and 「형님」 cannot be understood simply by translating them literally as “older brother” and “respected older brother”; the level of formality in the relationship also matters.
How Is 「누나」 Used? How Men Address Older Women
「누나」 is used by a man to address an older woman. It originally refers to a biological older sister or an older female cousin, but it can also be extended to an older woman the speaker knows well. A woman addressing an older woman uses 「언니」 instead, so 「누나／언니」, like 「형／오빠」, must be chosen according to the speaker’s gender.
- Example: 누나, 잘 지냈어요?
Romanization: nu-na, jal ji-nae-sseo-yo?
Translation: Noona, have you been well?
This is suitable when a man speaks to an older woman he knows well and sounds more familiar than using her name with an honorific. - Example: 사촌 누나가 부산에 있어요.
Romanization: sa-chon nu-na-ga bu-san-e i-sseo-yo.
Translation: My older female cousin is in Busan.
「사촌 누나」 refers to an older female cousin from the perspective of a male speaker. - Example: 누나라고 불러도 괜찮아요?
Romanization: nu-na-ra-go bul-leo-do gwaen-cha-na-yo?
Translation: Is it okay if I call you 「누나」?
Asking before using it for a non-relative helps avoid imposing a level of familiarity that the other person may not want.
The honorific form of 「누나」 is 「누님」. Among friends, 「누나」 is heard more often in everyday conversation, while 「누님」 carries a greater sense of respect and distance. Usage varies depending on the family, community, and relationship between the two people.
How Is 「선생님」 Used? It Does Not Only Mean “Teacher” at School
The most direct meaning of 「선생님」 is an honorific form of 「선생」, so teachers and instructors can be addressed as 「선생님」. In actual Korean, however, it can also function as a broader polite form of address, especially when you do not know someone’s name or job title and a familiar form of address would be inappropriate.
The 「-님」 in 「선생님」 is an honorific suffix expressing respect and serves the same general function as the 「님」 in 「사장님、부모님、형님」. In places such as public institutions, someone whose name is unknown may also be addressed directly as 「선생님」. If you already know the person’s job title, however, using a title such as 「과장님」 is usually more precise.
- Example: 선생님, 질문이 있어요.
Romanization: seon-saeng-nim, jil-mun-i i-sseo-yo.
Translation: Teacher, I have a question.
This is the most typical usage in a classroom or educational relationship. - Example: 김 선생님께 여쭤봤어요.
Romanization: gim seon-saeng-nim-kke yeo-jjwo-bwa-sseo-yo.
Translation: I asked Mr./Ms.
Kim.
A surname plus 「선생님」 can be used to address a teacher or another person for whom 「선생님」 is an appropriate respectful title.
The 「께」 in this sentence is an honorific particle and often appears together with forms of address that carry a higher degree of politeness. For more detail, see How to Use Korean 「께」.
- Example: 선생님, 이쪽으로 오시면 됩니다.
Romanization: seon-saeng-nim, i-jjok-eu-ro o-si-myeon doem-ni-da.
Translation: Sir/Ma’am, please come this way.
In service or public settings, 「선생님」 may be used as a polite way to address an adult whose name is unknown. In this case, it does not necessarily translate as “teacher.”
Can You Call Someone 「오빠／언니／형／누나」 the First Time You Meet?
When meeting someone for the first time, you normally would not immediately call them 「오빠／언니／형／누나」 simply because you know they are older than you. Although all four terms can be used for people who are not relatives, they carry a sense that the relationship has already become somewhat close, beyond simply meaning “older brother” or “older sister.” So when you have just met and do not yet know how the other person prefers to be addressed, it is more natural to use whatever form of address is appropriate for the situation first. That said, there is no absolute rule that these words can never be used at a first meeting. If you have already learned each other’s ages during the conversation and the other person says, “Just call me 오빠” or “Just call me 형,” or if you ask, “Can I call you that?” and receive permission, you can quickly switch to 「오빠／언니／형／누나」 even if you met only that day. So the real difference is not how many days you have known each other, but whether both people have already accepted this more familiar form of address.
▌Situation ①｜After Just Meeting, Asking Whether You Can Use 「오빠」
A：제가 한 살 어린데, 오빠라고 불러도 돼요?
je-ga han sal eo-rin-de, o-ppa-ra-go bul-leo-do dwae-yo?
I’m one year younger than you. Can I call you 「오빠」?
B：네, 편하게 오빠라고 불러요.
ne, pyeon-ha-ge o-ppa-ra-go bul-leo-yo.
Yes, feel free to call me 「오빠」.
📌 Addressing Point: Here, a woman is speaking to an older man. Even though she already knows he is one year older, she does not suddenly switch from the previous form of address to 「오빠」. Instead, she first asks 「오빠라고 불러도 돼요?」, meaning “Can I call you 오빠?” 「-라고 부르다」 means “to call or refer to someone as,” and adding 「-아／어도 돼요?」 allows you to ask whether using that form of address is acceptable. When the other person replies 「편하게 오빠라고 불러요」, he is indicating that he does not mind reducing the distance between them, so using 「오빠」 afterward sounds natural. If you are not sure whether someone you have just met is comfortable with this kind of address, asking is safer than simply using it.
▌Situation ②｜Starting With a General Form of Address, Then Being Told “Just Call Me 형”
A：민수 씨라고 부르면 될까요?
min-su ssi-ra-go bu-reu-myeon doel-kka-yo?
Would it be okay if I call you 「민수 씨」?
B：저보다 어리시니까 편하게 형이라고 부르세요.
jeo-bo-da eo-ri-si-ni-kka pyeon-ha-ge hyeong-i-ra-go bu-reu-se-yo.
Since you’re younger than me, feel free to call me 「형」.
A：네, 형. 알겠습니다.
ne, hyeong. al-ge-sseum-ni-da.
Okay, hyung. Got it.
📌 Addressing Point: This situation involves two men meeting for the first time. The younger person initially addresses the other as 「민수 씨」, and after the older person learns their respective ages, he voluntarily says 「형이라고 부르세요」, indicating that 「형」 is acceptable. At that point, even though the two have only just met, it is not strange for the younger man to call him 「형」 from then on. Conversely, if the other person has not suggested it and they have not yet discussed how they would like to address each other, calling someone 「형」 simply because “he is older than me” can sometimes sound as though you are placing the relationship into the category of familiar acquaintances too quickly. The same applies to 「오빠、언니、누나」: they are not completely impossible at a first meeting, but the two people normally establish or confirm the form of address first.
▌Situation ③｜Using 「선생님」 in a Formal or Public Setting When You Do Not Know the Person’s Title
A：선생님, 이 서류는 어디에 제출하면 되나요?
seon-saeng-nim, i seo-ryu-neun eo-di-e je-chul-ha-myeon doe-na-yo?
Excuse me, where should I submit this document?
B：저쪽 창구에 제출하시면 됩니다.
jeo-jjok chang-gu-e je-chul-ha-si-myeon doem-ni-da.
You can submit it at the counter over there.
📌 Addressing Point: If the person you are meeting for the first time is someone you encounter at a company, bank, government office, or similar setting, you cannot simply use 「오빠／언니／형／누나」 because they look older than you. These forms of address relate not only to age but also to the relationship between the two people. In formal settings, forms of address are chosen according to job title, status, and degree of politeness. If you know the other person’s title, you can use a form such as 「과장님」, meaning “title + 님.” If you do not know their name or title, 「선생님」 may be heard in some public or formal settings. Here, 「선생님」 does not mean that the person’s occupation is teaching; it is simply being used as a polite way to address them.
Additional Usage Notes:
- 「오빠／언니／형／누나」 are not simply labels for “a man or woman older than me.” A woman uses 「오빠」 for an older man and 「언니」 for an older woman, while a man uses 「형」 for an older man and 「누나」 for an older woman. But matching the gender and age conditions only means the form is structurally possible; whether people actually use it also depends on how familiar they are with each other. So when you have just learned someone’s age, there is no need to change the way you address them immediately.
- It is also not absolutely forbidden to use 「오빠／언니／형／누나」 the first time you meet. If the other person directly says 「오빠라고 불러요」「편하게 언니라고 해요」「형이라고 부르세요」, they are telling you that you can use that form of address. You can also ask 「오빠라고 불러도 돼요?」「언니라고 불러도 돼요?」 yourself. Once you have permission, you can switch directly to 「오빠／언니／형／누나」 even if you have only met that day. So “you cannot use these the first time you meet” is better understood as a usage tendency intended to avoid becoming too familiar too quickly, not as an inviolable grammatical rule.
- In formal situations, work relationships, or service settings, do not choose a form of address based solely on age. If you know the person’s job title, it is usually possible to use “title + 님,” such as 「과장님」. If you know the person’s name, whether 「씨／님」 is appropriate depends on your respective positions and the situation. If you know neither the person’s name nor their title, 「선생님」 may be used as a polite form of address in some public settings. In other words, 「오빠／언니／형／누나」 tend to appear in interactions where the relationship is already relatively familiar or close, whereas 「선생님」 manages a different kind of polite distance. The two cannot be interchanged simply according to how old the other person looks.
How Are Korean 「오빠、언니、형、누나」 Different From Japanese 「お兄さん／お姉さん」?
When translating Korean 「오빠、언니、형、누나」 into Japanese, it is easy to match them directly with 「お兄さん／お姉さん」, but the two languages divide forms of address differently. Korean first considers whether the speaker is male or female: a woman calls an older man 「오빠」 and an older woman 「언니」, while a man calls an older man 「형」 and an older woman 「누나」. Japanese 「お兄さん／お姉さん」 do not split into four different words according to the speaker’s gender. Both men and women can use 「お兄さん／お姉さん」. Japanese also distinguishes between addressing your own family member directly, referring to your own older brother or sister when speaking to an outsider, and addressing someone who is not related to you. So you cannot simply see 「오빠」 and always convert it into 「お兄さん」, or see 「언니」 and directly translate it as 「お姉さん」.
◆ Japanese Form of Address: 「お兄さん」
Japanese 「お兄さん」 can be used to address an older brother and, in some situations, a man who is not related to you. However, it does not work exactly like Korean 「오빠／형」. In Korean, when addressing the same older man, a woman uses 「오빠」 and a man uses 「형」; Japanese does not switch to a different word depending on the speaker’s gender. However, when referring to your own older brother while speaking to someone outside the family, Japanese normally uses 「兄（あに）」 rather than 「お兄さん」. So if the Korean sentence 「우리 오빠는 회사원이에요」 is being used to tell someone about your own older brother, the more natural Japanese version would be 「私の兄は会社員です」. You cannot simply replace 「오빠」 with 「お兄さん」 word for word.
- Example: お兄さんは今、家にいますか？
Kana: おにいさんは いま、いえに いますか？
Translation: Is your older brother at home now?
Here, 「お兄さん」 refers to the other person’s older brother. In Korean, if you are asking about a woman’s older brother, you can say 「오빠는 지금 집에 있어요?」. If you are asking about a man’s older brother, it becomes 「형은 지금 집에 있어요?」. Looking only at the Japanese word 「お兄さん」 does not allow you to determine the speaker’s gender in the same way Korean does. - Example: 私の兄はソウルで働いています。
Kana: わたしの あには ソウルで はたらいています。
Translation: My older brother works in Seoul.
This sentence refers to the speaker’s own older brother when speaking to someone outside the family, so 「兄（あに）」 is used. In Korean, a female speaker would say 「제 오빠는 서울에서 일해요」, while a male speaker would say 「제 형은 서울에서 일해요」. This is another clear difference between the two languages: Korean still chooses between 「오빠」 and 「형」 according to the speaker’s gender even when referring to one’s own older brother, while Japanese mainly distinguishes between addressing one’s family within the family and talking about one’s family to outsiders. - Example: お兄さん、これ落としましたよ。
Kana: おにいさん、これ おとしましたよ。
Translation: Sir / hey, you dropped this.
「お兄さん」 can also be used to call out to a young man who is not related to you, for example when you do not know his name and use the term according to his apparent age and the situation. However, this kind of 「お兄さん」 carries a different relational meaning from Korean 「오빠／형」. Korean 「오빠／형」 usually suggests some degree of familiarity between the two people, so you cannot use it in every encounter with a stranger simply because the person looks older than you.
◆ Japanese Form of Address: 「お姉さん」
「お姉さん」 is used similarly to 「お兄さん」. It can refer to the other person’s older sister, be used to address your own older sister directly, and in some situations be used for a woman who is not related to you. It likewise does not divide into two words according to the speaker’s gender, so both Korean 「언니」 and 「누나」 can correspond to Japanese 「お姉さん」. When referring to your own older sister while speaking to an outsider, Japanese normally switches to 「姉（あね）」. This means there is no one-to-one memorization system where “언니 = お姉さん” and “누나 = some other Japanese word.” Korean focuses on who is addressing whom, while Japanese has its own distinction between forms used within the family and those used when speaking outside the family.
- Example: お姉さんは大学生ですか？
Kana: おねえさんは だいがくせいですか？
Translation: Is your older sister a university student?
Here, 「お姉さん」 refers to the other person’s older sister. In Korean, if the person you are speaking to is female, she would call her own older sister 「언니」. If the person is male, he would use 「누나」. So a single Japanese 「お姉さん」 may require different Korean words depending on the relationship between the people involved. - Example: 私の姉は日本に住んでいます。
Kana: わたしの あねは にほんに すんでいます。
Translation: My older sister lives in Japan.
When referring to your own older sister while speaking to someone outside the family, Japanese normally uses 「姉（あね）」. In Korean, a woman could say 「제 언니는 일본에 살아요」, while a man could say 「제 누나는 일본에 살아요」. Korean retains the distinction based on the speaker’s gender, whereas Japanese 「姉」 does not change according to whether the speaker is male or female. - Example: お姉さん、注文してもいいですか？
Kana: おねえさん、ちゅうもんしても いいですか？
Translation: Miss, may I order?
Japanese 「お姉さん」 may also be used to address a relatively young woman who is not related to the speaker and does not necessarily mean that the speaker literally sees her as an “older sister.” Korean 「언니」 may also appear among unrelated women in certain shops, interactions with regular customers, or relatively familiar situations, but this does not mean that the two terms have exactly the same range of use. This kind of Japanese 「お姉さん」 may simply be chosen according to the other person’s apparent age or the situation, whereas Korean 「언니」 usually also communicates that the speaker is female and that the relationship between the two is relatively close.
◆ Your Own Older Brother or Sister: The Difference Between Japanese 「兄／姉」 and Korean
Japanese has a distinction in family terms that does not map perfectly onto Korean. When speaking directly to an older brother or sister, you can call them 「お兄さん／お姉さん」, and forms such as 「お兄ちゃん／お姉ちゃん」 are also common within families. But when referring to your own older brother or sister while speaking to someone outside the family, you normally use 「兄（あに）／姉（あね）」. Korean 「오빠／언니／형／누나」 do not necessarily switch to a different set of kinship terms depending on whether you are “speaking directly to the person” or “talking about that person to someone else.” For example, a woman can call her older brother 「오빠」 directly and can also say 「우리 오빠」 when talking about him to a friend. A man uses 「형」. Both languages adjust forms of address according to who is involved in the conversation, but they make that adjustment in different places.
- Example: 兄は三歳上です。
Kana: あには さんさい うえです。
Translation: My older brother is three years older than me.
If a woman says this in Korean, she can say 「오빠는 저보다 세 살 많아요」; a man can say 「형은 저보다 세 살 많아요」. The Japanese word 「兄」 itself contains no information about the speaker’s gender, whereas Korean 「오빠／형」 directly encodes that information in the form of address. - Example: 姉と一緒に旅行に行きました。
Kana: あねと いっしょに りょこうに いきました。
Translation: I went on a trip with my older sister.
In Korean, a woman can say 「언니랑 같이 여행 갔어요」, while a man uses 「누나랑 같이 여행 갔어요」. In Japanese, once you know the person is the speaker’s own older sister, 「姉」 is sufficient. In Korean, you also need to know whether the speaker is male or female.
◆ The Difference From 「오빠／언니／형／누나」:
The biggest difference between Korean 「오빠／언니／형／누나」 and Japanese 「お兄さん／お姉さん」 lies in how the two languages encode interpersonal relationships into forms of address. Korean directly divides the system into four words according to the speaker’s gender and the other person’s gender: a woman uses 「오빠」 for an older man and 「언니」 for an older woman, while a man uses 「형」 for an older man and 「누나」 for an older woman. Japanese has no equivalent four-way distinction. Both men and women may use 「お兄さん／お姉さん」, so looking only at those two Japanese terms normally does not tell you the speaker’s gender.
In addition, when Japanese speakers refer to their own older brother or sister while speaking to an outsider, they normally use 「兄／姉」, which differs from 「お兄さん／お姉さん」 used when directly addressing the person. Korean can continue to use 「오빠／언니／형／누나」. Both languages may also extend words originally associated with older brothers and sisters to people who are not related by blood, but the interpersonal distance is not exactly the same. Korean 「오빠／언니／형／누나」 often also reflect whether the two people are already familiar with each other and whether both accept this relatively intimate form of address. Japanese 「お兄さん／お姉さん」, by contrast, may sometimes be used simply according to the other person’s age or the immediate situation. When comparing the two systems, it is more accurate to first look at the speaker’s gender, the relationship between the people involved, and whether someone is addressing the person directly or referring to their own family, rather than memorizing fixed equations such as 「오빠 = お兄さん」 and 「언니 = お姉さん」.
The easiest mistake to make with Korean forms of address is to see the Chinese words for “older brother” or “older sister” and immediately substitute a single Korean word. In real conversations, the same older man may be called 「오빠」 by a woman and 「형」 by a man. The same older woman likewise becomes 「언니」 or 「누나」 depending on the speaker. These words also carry a certain degree of familiarity, so meeting the age requirement does not mean they can automatically be used in every kind of relationship.
「선생님」 should not be memorized as part of the same “older brother / older sister” group. It is closer to a polite form of address and, in addition to teachers, may also be heard in some public or formal situations. When you have just met someone and are not yet sure what form of address is appropriate, there is no need to rush into calling them 「오빠、언니、형、누나」. Waiting until the age relationship and interpersonal distance are clearer—or simply asking, “Is it okay if I call you this?”—is actually much closer to how Korean forms of address are used in practice.
FAQs
In standard usage, 「오빠」 is used by a woman to address a man older than herself. A man addressing an older man normally uses 「형」. It can also be used for someone who is not a family member, but generally only when there is a certain degree of familiarity.
Both can refer to an older woman. The difference lies in the speaker’s gender. A woman uses 「언니」 for an older woman, while a man uses 「누나」.
Both can refer to an older man. 「형」 is used by men, while 「오빠」 is used by women. Whether they can be used for someone who is not a relative also depends on how familiar the two people are with each other.
No. 「오빠」 itself is a form of address used by a woman for an older man. An older boyfriend may also be called this, which is why the term often appears in conversations between couples in Korean dramas and films, but the word itself does not mean “boyfriend.”
No. Teachers are the most typical people addressed as 「선생님」, but in some formal or service settings it can also be used as a polite way to address an adult whose name or job title is unknown. If the person’s title is known, using 「job title＋님」 is usually more precise.