The club's longest serving player and record appearance holder at the Stadium of Light is now committed to Sunderland until 2027. The Black Cats have the option of a further 12-month extension.
O'Nien played an integral role in Sunderland's promotion last season, making 48 appearances throughout the campaign before being forced off injured in just the second minute of the play-off final.
The defender will miss the start of the Premier League season after undergoing shoulder surgery in the summer, but the 30-year-old, who joined the Black Cats when they were in League One can't wait for his first taste of top flight football.
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He 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.”
Kristjaan Speakman, sporting director, said: “Luke has been integral to our progress throughout the last four years, both on and off the field.
"His passion for Sunderland is clear and infectious, and there are many reasons why he is so highly regarded by our supporters and staff, and his team-mates.
"This contract extension is recognition of his contribution to date, whilst reaffirming our expectations moving forward. We’ve thoroughly enjoyed working with him and now look forward to building the next stage of our shared journey.”