1. What does it mean?
올팬 combines English all with 팬, fan. It describes supporting the whole group's membership rather than following just one member. Someone may still have a favorite while identifying as an 올팬.
Also written or said as olpaen
2. The nuance
It describes a person's approach to fandom, not a promise to buy every release or agree with everything the members do.
3. Examples
“멤버들 다 좋아해서 나는 올팬이야.”
I like all the members, so I consider myself a whole-group fan.
“올팬이라 단체 사진을 배경으로 해 놨어.”
I support the whole group, so I made a group photo my wallpaper.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.