Hey, it never get's translated or given a translation note, what the fuck is an oshi
So, the word oshi literally means push, but in the context of the slang, it's someone you push for, or support, or promote. It's either a fictional character or an actual person (it can be a musician, or athlete, or actor, or livestreamer, or even a porn star) that you want to see succeed and achieve their goals, often your favorite out of a group (in the case of this manga, MC's favorite character out of an entire cast of characters). The name oshi suggests you like them in a way that you can tangibly demonstrate, by buying their merchandise, or making fan art about them, or subscribing/liking all of their videos, or watching their livestreams and giving them money via superchats or tips or whatever, or buying tickets to see them in person at a live concert or sports game, or simply recommending them to others. So yeah, the closest translation of oshi would be favorite, but it goes a bit further than that because you don't just like them more than others, you've formed an emotional attachment to them, sometimes to the point of developing a
parasocial relationship.