As I said up thread, diving is yellow card whenever it happens. Refs shouldn't be letting it go for 80 minutes before doing something about it.
The reason Oliver had such a terrible game is because he let far too much go until the very end.
On top of the diving, he let a few players get away with repeated fouls without further punishment, or left the punishment until it was far too late, and bottled second yellows for Bellerin, Ramsey and Rojo (and probably some others by the time he'd let all the other fouls go), which is why the end of the match was such a farce. Had he booked Welbeck/Sanchez/Di Maria for diving when they first did it, we wouldn't be having this discussion because none of them would have been reacting to the card when tempers were running high and the match was very tense, and other players would have been wary of trying it on for the remainder of the game.
Likewise, had he booked Fellaini after his first 3 fouls, rather than waiting for him to commit a few more before doing anything about it, and flashed a yellow card to a couple of other players when they'd done the same, there wouldn't have been so many daft tackles flying around towards the end of the match. Also, had he not set a precedent for cynical fouls being met with yellow cards, there wouldn't have been the uproar about him not sending Bellerin off, which again, wouldn't have raised the intensity of the match. Rojo probably should have seen red too by the end of the game, but again, this was all caused by Oliver losing control of the game.
On top of that there were a couple of occasions where he blatantly didn't enforce the rules. Monreal deliberately handballed it to stop a decent attacking move but wasn't punished (he was one who should have already been on a yellow), and Ramsey got booked for hauling Young down, but was let off for kicking the ball away.
By all accounts, it was a terrible refereeing performance, riddled with inconsistent officiating, apparent ignorance for the laws of the game, and a general lack of control over the match. The fact that so many saw no problem with it, and in fact thought it was a good performance is quite worrying. I don't think it would have made a jot of difference to the final score given that United were woeful in the second half, but that doesn't mean that Michael Oliver had a good game as referee.