This is a way to address a male individual that is older than you. 오빠 (o-bba or oppa) is used by women, and 형 (hyeong or hyung) is used by men.
This is how we address a female individual that is older than us. Just like above, 언니 (eon-ni or unni) is used by women, and 누나 (nuna or noona) is used by men.
오빠, 형, 언니, and 누나 are relatively casual ways to address someone. Use them only when you are close with the person, and do not use this with people that are a lot older than you are (I personally recommend you to use this with people that are at most 10 years older).
You can also attach these to names. When the name is Korean and ends in a consonant, use 이 (i) as a connector.
Example:
지아 (Ji-Ah) 누나
윤우 (Yun-U) 오빠
이슬 (Yi-Seul)이 언니
정영 (Jung-Young)이 형
This is how we address people that are in the same organization as you (e.g. school, work) and are more senior than you. You may have come across this phrase in Korean dramas - used commonly between colleagues and students.
Adding 님 (nim) makes it more polite and formal. We recommend starting off with 선배님 (seon-bae-nim), and dialing it down to 선배 (seon-bae) once you are close with the person.
You can also attach these to names, but you don't need to use any connectors. A simple name +선배 or 선배님 will do the job!
This is teacher in Korean, and also how we address them.
As a word on its own, you can attach subjects they teach or the organizations they teach. When you address someone with this word, it is relatively less common to attach it to names, especially if you're a student addressing your teacher.
Fun fact - a slang abbreviation for 선생님 (seon-seng-nim) is 쌤 (ssam), and you can use this with teachers that you are comfortable and close with!
선생님(seon-seng-nim) is also used to address people that are not particularly teachers - take a look at this article for a deep dive into the word 선생님 (seon-seng-nim).
This word translates to boss. Employees of businesses can call their boss with this word.
Another fun fact - this is one of the ways to address a staff in a restaurant (especially in less fancy ones). Even if the person you are calling is seemingly not the boss of the place, it is okay to call him/her with this word.
Take a look at this article for more phrases to know when you're in a restaurant!