O'Nien has been absent since late November after having surgery on his shoulder, but travelled with the matchday squad to Charlton Athletic and took a full part in the warm up on Saturday.
Head coach Alex Neil has confirmed that the 27-year-old will be in contention when Fleetwood Town visit the Stadium of Light on Tuesday night, and the versatile midfielder is confident that the team he is returning to is on the up.
"I'm feeling good," O'Nien told The Echo.
The strongest Sunderland XI Alex Neil could pick right now - and the clear transfer priorities it shows
Nine goalkeepers Sunderland could try to sign this summer - including Chelsea youngster, Nottingham Forest stopper and ex-Wolves man
Sunderland transfer news: Black Cats face competition for Derby County playmaker, Cardiff City eye move for former Stoke City loanee
Contract details revealed as Sunderland star Anthony Patterson signs new deal in massive boost
Championship promotion odds: Sunderland's pre-season chances compared to Middlesbrough, Norwich, West Brom and league rivals
"I'm back amongst it now so I've timed it nicely for the hunt for promotion.
"I'm raring to go.
"It's been difficult [watching on], I've had some time with my family and chased my little one around the house, which I think counts as a pre-season!
"I'm back fit and looking forward to it.
"You've just got to look at the last two games, how solid we've been at the back, giving the platform for our forward players, we're in a good spot now so we've got to go and build on it."
O'Nien was speaking at The Ramside hall as he collected his Sir Bobby Robson Foundation Sports Personality of the Year award for 2020.
O'Nien joins former Sunderland strikers Jermain Defoe and Niall Quinn as a winner of the award.
The 27-year-old was recognised for hsi work helping to keep fans active via youtube fitness videos during the height of lockdown, which he also used to raise more than £7,000 for NHS Charities Together.
Sir Bobby Robson launched his Foundation in 2008. Part of Newcastle Hospitals Charity, it has gone on to raise over £16m to help find more effective ways to detect and treat cancer, working within the NHS and in partnership with other leading charities and organisations.
O'Nien dedicated his award to key workers and NHS staff, and said it was a huge honour to join such a presitigious list.
He has been an immensely popular figure on Wearside since his arrival, and spoke of the affection both he and his family have for the region.
"I love being in the North East," O'Nien said.
"Everyone has been so welcoming to me and my family so to win an award like this, it's genuinely a big honour.
"My little one is a northerner, everyone is great to us. We've made our home here, and all the fans from the moment I've joined have been nothing but supportive. I love putting that red-and-white shirt on and I'm looking forward to the end of the season.
"You see the people who have won this trophy before, even just to be mentioned in the same sentence is an honour, let alone to win the award.
"In terms of the award, I just wanted to try and help during a tough time, the NHS workers and everyone on the frontline are the true deservers of this, and it's their award because they inspired us during that tough period."