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.