The defender is now committed to Wearside until 2027, with an option for another year. O'Nien is Sunderland's record appearances holder at the Stadium of Light.
O’Nien said: “This is another huge honour for me. I’ve had the pleasure of being part of a side that has achieved two promotions, and there are so many other memories to recall.
"I’m so happy to be given the chance to create more by extending my stay. Everyone knows how much pride I hold in representing this Club. Our journey to the Premier League is something that will stay with me for the rest of my life, but the hard work doesn’t stop, and I can’t wait to take on this new challenge.
"I can’t thank the city and the Club enough for the way they’ve embraced me and my family. This has been our home for many years, and there is no other place in the world that we want to be.”