You can wear pretty much whatever you want that isn't expressly black tie. If you don't directly seek out formal events, you probably won't need to attend any other than formal hall, for which you can probably wear anything. Even if an event is designated black tie, I doubt anyone would say anything if you were something that wasn't quite black tie standard. Sunday formal at Magdalen is supposedly black tie, but I've frequently seen things like skirt+blouse and suits that are very far from black. If you're just trying to get away with something, you won't have any issues, but if you're attending a ball or something of the sort, a dress would probably be recommended.
Don't worry about the bop at all. I feel like getting the freshers to dress up is almost a prank, and absolutely nobody cares if your ensemble isn't spectacular. By the end of the year, we had about 30% turnout on bops, and maybe a quarter of attendees actually dressed up.