The Irishman, 21, who impressed on loan at City last term, has linked up once again with Bantams chief Gary Bowyer, who he also previously worked with at Blackpool.
O’Connor, handed the captain’s armband at the end of last season and Bradford’s eighth close-season recruit, said: “This is the best possible move for me.
“Gary and I have a great relationship. We compliment each other well and I understand his demands.
“Over the coming three years, I am desperate to be associated with success. I will do all I can to help us get promoted.
Doncaster Rovers officials have stressed that they have had no contact from Hull City regarding Grant McCann and insist that they would not welcome any approach either after the Rovers chief rose to the top of the betting for the vacant Tigers post.
Meanwhie, Barnsley co-chairman Paul Conway remains hopeful that the club will secure the futures of several further leading players after Cameron McGeehan signed a new two-year deal.
The 24-year-old is now contracted at the club until the summer of 2021, following in the footsteps of Jacob Brown and Cauley Woodrow, who agreed extensions earlier on in the close season.
Barnsley remain in talks with influential defender Ethan Pinnock – whose current deal expires next June – regarding fresh terms.
Conway said: “We are pleased to see Cameron sign a new contract with us. It is a well-deserved reward for how well he played last season, which was further recognised with him being included in the EFL Team of the Season.
“This is part of our stated strategy, to develop young players and extend their deals for as long as possible. We have seen Jacob Brown, Cauley Woodrow and Mike Bähre recently further their stays as well, so now we hope to see more players do the same.”
McGeehan added: “Off the back of last season, I want to continue the momentum.
“We had a really good season, it was great for everyone at the club to enjoy that, so I wanted to commit my future and carry on the journey.”