Meaning of “Oshi(推し)”
“Oshi(推し)” means a person who I support.
If you are someone’s big fan, it means your oshi(推し) is the someone.
And if someone ask you “Dare oshi(誰推し)?”, it means “Who is your oshi(推し)” or “Who’s your favorite?”.
Then you should answer “[Someone’s name you love and support] oshi(推し)”.
I’ll give you additional information. You can use these if you like “group” something.
“Hako oshi(箱推し)” means you’re a fan of whole members of the group.
“Hako(箱)” means box.
“Oshi mem(推しメン)” means a member you support.
In this case, mem(メン) is an abbreviation for member(メンバー).
By the way, “Oshi(推し)” as a noun sounds like a slang, maybe Otaku’s slang just a bit, but as a verb like “Oshi te iru(推している)” or “Osu(推す)” is not a slang.
A: Actually, I’m morota(モーヲタ) too.
(Morota(モーヲタ) is an abbreviation for モーニング娘ヲタク(Morning musume otaku), “Morning musume” is a Japanese Idol group.)
B: Seriously? You should have told me earlier! Dare oshi(誰推し)?
A: Marchan oshi!! Dare oshi(誰推し)?
B: I used to have a oshi mem(推しメン), but now I’m hako oshi(箱推し).