“Oppa” literally means elder brother (for a woman) but it is widely used by women to address any older male friend. In particular, Korean girls always call their boyfriends “oppa”. I guess the idea behind this is to indicate that he is as close as a brother. Or even closer:-)To call a guy an oppa is basically to call him a close friend, possibly a boyfriend. As you can tell, the concept of “oppa” doesn’t translate that easily into Western culture.

[Part III] So if you click into the hyperlinked term 자매 to arrive the following webpage: en.wiktionary.org/wiki/%EC%9E%90%EB%A7%A4#Korean , then you will see the following "similar terms" (hyperlinked): 누나 , 언니 , and 여동생 . The first term, 누나 (nu-na), refers to the plain form to address an elder sister of a MALE speaker. Its corresponding honorific form is 누님 (nu-nim). The second term, 언니 (on-ni), refers to the plain form to address an elder sister of a FEMALE speaker. Its corresponding honorific form is 언님 (on-nim). For the third term, 여동생 (yeo-dong-saeng), please kindly refer to the term 동생 as explained previously. (N.B. There are no given explanations on the honorific terms for younger brothers and younger sisters. I presume, however, that one may transform them into the honorific counterparts by adding the honorific suffix 님 onto the corresponding terms.) Well, I hope that the above explanations may give you and other readers some ideas for the given term in question. In addition, for the readers who have certain Korean (and even some Japanese) background, I suggest NAVER online dictionary website, available at: dic.naver.com , for consultation of a typical Korean vocabulary.

[Part II] The fourth term, 동생 (dong-saeng), originated from the Chinese characters 同生 , means a YOUNGER brother OR sister of oneself (regardless gender of the speaker). Should one want to further emphasize the gender of this younger brother or sister, then the term 남 (nam, meaning "male") or 여 (yeo, meaning "female") is adopted and placed before the term 동생 . Hence, 남동생 = younger brother, and 여동생 = younger sister. The fifth term, 아우 (a-u), is a plain form to address an elder brother of a MALE speaker. Now, if you type "sister" on the entry of wikitionary, your browser should arrive the following webpage: en.wiktionary.org/wiki/sister Again, you expand the column of "woman or girl having the same parents" under "Translations", but unfortunately the English wikitionary does not provide so much in details as the ones for the term "brother". Anyway, in Korean, it states the term 자매 (ja-mae). The term, which comes from the Chinese characters 姉妹 , is a general term that literally means "elder-and-younger sisters" (It should be however noticed that in modern Chinese, elder sister is written as 姊 or 姐 , but never as 姉 .)

[Part I] These days it came to an occasion, where I try to self clarify the related terms of brothers and sisters (based on the identity of the speaker) in both Korean and Japanese. I looked up the wikipedia dictionary, available at: en.wiktionary.org/wiki/Wiktionary:Main_Page , and I obtained the following information. If you type "brother" on the entry of wiki(dic)tionary, your browser should arrive the following webpage: en.wiktionary.org/wiki/brother Then you go to expand the column of "male sibling" under "Translations", and you will see the respectively translations in different languages. In Korean, it states the followings: 형제 (hyeong-je), 형님 (hyeong-nim), 오빠 (o-bba), 동생 (dong-saeng), 아우 (a-u) [I particularly placed a hyphen in the romanized spelling to split out between each korean hangeul.] The first term, 형제 (hyeong-je), which comes from the Chinese characters 兄弟 , is a general term that literally means "elder-and-younger brothers". The second term, 형님 (hyeong-nim), is an honorific form to address an elder brother of oneself (when no third party is around), or to someone who is respected as his own elder brother, or an elder brother of someone, e.g. partner of the speaker. I expect this term is effective to a speaker in both genders, i.e., male and female, but I am not fully sure of this. The third term, 오빠 (o-bba, or o-ppa as given in your question), is a plain form to address an elder brother of a FEMALE speaker. This is a characteristic in Korean culture, where an elder brother is called DIFFERENTLY between a MALE and a FEMALE speaker. In contrary, the plain form to address an elder brother of a MALE speaker is 형 (hyeong) . This is very similar to the second term, because the suffix 님 (nim), as shown previously, is a general practice to transform an addressing into an honorific form in Korean language.