As I already mentioned, the main character of mha is Izuku Midoriya. That's set in stone and so you shouldn't go out of your way to make your oc the main character. It's alright to be a key character in the story, but your oc isn't the protagonist, keep that in mind. I think I already put down the main thing to not do in this paragraph so let's just start.

Your oc's role

It's alright to make your oc a part of the events that occur throughout mha but if they're in almost every single episode as a key character, it's going to get annoying. Think about their role and whether they would be there or not. I'll write out some examples to help you.

USJ Incident - Should only be there if they're one of the students, a villain or one of the teachers called in for backup.
I Island Incident - Should have only been inside the tower if they were one of the villains or if they had a reason they got to the premier early.
UA Sports Festival- Should only be there if they're a student or a spectator in the crowd.
Saving Eri  - Should only be there if they were one of the students who went to work studies or if they were a Pro Hero.

I'll use the I Island Incident as my key example. The students who got into the premier early had a solid reason why. For example, Todoroki, Iida and Midoriya were there because of Pro Heroes while Momo's family supplied money towards the island and so she was allowed to go along with two guests, Jirou and Uraraka. If you think your oc would fit into the situation, find a reason why they were there in the first place.

Your oc doesn't need to be in every single scene for them to be important, just remember that.