The meaning of OPPA
Main article: What does OPPA mean?
In Korea, Oppa is an honorific title used by younger girls/women to address older boys/men. To a certain extent, it is a respectful way to show who is the elder. Usually this happens when both parties achieve a certain relationship.
So, in short, Oppa is something like "big brother".
Calling Idols by OPPA
It is also acceptable, although there is still some discussion, for fandom to call idols by oppa. The reason is that fandom and idols already have somewhat a close relationship, whether it's personal or not, which they consider themselves a family.
Since Korean culture takes age and respect very seriously, oppa should not be used to address someone younger. Otherwise, this might demean the title or cause some embarrassment. In fact, on BTS' fan cafe you have to call the staff and members by the correct term or you could be banned.
Even so, many K-pop fangirls started to call their younger male idols by oppa too. Probably not out of disrespect, on the contrary, just to show their passion. As the Korean saying:
"If you are handsome, you are my OPPA!"
The idols usually don't consider this rude and even encourage such behavior, with only a few exceptions. Curiously though, the fans themselves are the ones who complain the most: either due to the improper age, romantic or sexual connotation, non Korean speech or weird foreigners (close to a koreaboo), ... etc.
BTS members who are often called OPPA
Full name: Jeon Jung-kook
Jungkook is the official maknae followed by Taehyung and Jimin in the maknae line. He was born in 1997 and debuted in BTS as a teenager in 2013. At that time, a lot of his fans were older than him and obviously he was too young to be called oppa.
Evolution of BTS's Jungkook from pre-debut in 2013 to 2017
Even so, in 2014, BTS released the album "Skool Luv Affair" which the music video of the lead single "Boy In Luv" shows him saying to a school girl:
"I want to be your oppa", Jungkook
Scene from BTS's Boy in Luv music video 
However, contrary to what the lyrics say, at a fan sign event in 2016, he asked the audience not to call him oppa:
[Jungkook] But I'm not your oppa.
[Fans] Oppa! Jungkook-oppa!
[Jungkook] You keep calling me oppa.
And even asked his fans to bring ID cards to prove they are indeed younger than him before calling him oppa:
[Jungkook] All of my ARMY, from now on bring your identification cards.
This single event triggered several rumors that he actually hates when fans much older call him oppa (although the video doesn't show their ages). However, this is an overstatement. He may have been annoyed with the situation but wasn't rude, in fact he was smiling politely all the time. Besides, the mood of the event was clearly a playful one and even J-Hope teased his orders:
[J-Hope] Yeah, everyone bring your ID cards and show us your age. [...] If you don't have an ID, call us oppa.
After that, he was called several times oppa and didn't appear to be upset:
In 2018, at a fan event, he gave the famous finger heart to the audience when called oppa:
Jungkook doing the finger heart for fans after being called oppa in 2018
In 2019, at the M2U fan sign, fangirls called him oppa continuously for the entire 40 min event. Even so, he just kept focus on his autographs and did not show any sign to be upset.
These led rumors that, after years protesting, Jungkook finally gave ARMY permission to call him oppa.
Full name: Kim Tae-hyung
Evolution of BTS's V from debut in 2013 to 2019
Taehyung is known for his easy-going traits, meaning that he doesn't get bothered when fangirls much older called him oppa.
He even told a fan to tell some bullies that he was her boyfriend. It was at a 2018 BTS fan signing, and Taehyung could be seen telling one lucky fan something that made the others envy:
"You look like a lot of guys would want to contact you, am I right?", Taehyung
He then became quite protective and told her the sweetest thing, in the case anyone ever bothered her:
"If you think they aren't that great, or if you don't really like them, give them oppa's [my] name", Taehyung
However, he does notice when younger girls don't use oppa to call him. For example, during a live broadcast, he was reading a fan's plea for advice on her current situation:
"I am a high school student and so nervous about my job interview.
What should I do, Taehyung-ah? What do you do when you're nervous?", fangirl
Taehyung during a live broadcast
Immediately, he questioned about the honorific term she was using:
"You're a high school student … I wonder why you're calling me Taehyung-ah? I'm 23 years old.", Taehyung
Taehyung was trying to understand what would have prevented her from using the correct respectful term oppa for an older men. He finally concluded that she must be living in America since Koreans put much more emphasis on age difference than Americans do:
"You must be living in America. In America there is no appellation for the elders. In America they do that.
I like the American style.", Taehyung
Hence, these terms of respect are built into the Korean language for a reason.
[ 1 ] "[MV] BTS - Boy In Luv", YouTube Video
[ 2 ] "I'm not your oppa!", YouTube Video
[ 3 ] "When someone calls jungkook oppa", YouTube Video
[ 4 ] "M2U BTS JUNGKOOK FOCUS FAKE LOVE", YouTube Video
[ 5 ] "BTS V Live", YouTube Video