If Hamer does go then this surely would be a deal Wilder will want to do. He's only been on the bench for Boro of late, Carrick has also been coy on whether they'd bring him back. I think if United want him O'Brien would want to come although O'Brien has said he would definitely go back if the offer came from Boro but I also suspect that he would say that if asked because it's the standad thing to say. Even BBD has said similar for us and we all know there's no chance of that!
In terms of Forest not wanting him to join us, surely that only applies when we were in the same league as them, if they stay up I'm sure the'd be happy to loan him to us or sell. It would probably be a loan knowing United but he would be someone we can realistically sign as opposed to recent loan deals where the players were younger and valued at £20m plus, and Doyle was still one of those despite low fee because of the Wolves playing going the other way to City.
O'Brien has had a serious injury though last season, he seems over that now but I think that might put Wilder off a little.