The 27-year-old has rejoined the club he once played for as a teenager for an undisclosed fee on a two-and-a-half year deal.
Speaking to the official site O’Neil said, “I am really pleased to sign. It all happened very quickly. I played for Middlesbrough at the weekend and didn’t know anything was going to happen. It has all gathered pace since then. There was also a big rush to do it quickly when they realised I wasn’t cup-tied.
“I enjoyed my time at Middlesbrough, the fans and staff were really good but it has been difficult time on the pitch. It is nice to be back in the Premier League and play at the top level, which everyone wants to do. I am London born so it is good to come home.