Manager Jack Ross has been busy in the transfer market since making the switch to the Black Cats from St Mirren, and 10 signings have now been made ahead of the new season.
O'Nien has scored 16 goals from 119 appearances in three years at Wycombe, but the player has admitted that he jumped at the opportunity to join the North-East outfit.
The 23-year-old told the club's official website: "It's a huge honour because it's such a big club. The last few days have been pretty manic for me and it's all happened so fast, but I'm delighted to be here, and I can't thank the club enough for giving me the opportunity.
"The direction of the club is only going to be up, and I'm going to do everything I can to help get the club to back where it belongs."
O'Nien could feature in the squad when Sunderland face Charlton Athletic in their League One opener this weekend.