The 30-year-old, who made 24 Premier League appearances for West Ham last season, moves to Loftus Road on a free transfer and has signed a one-year deal with the club
The 30-year-old has signed a one-year deal with Harry Redknapp's side after his contract with West Ham expired earlier this summer.
“Gary’s a good player and a good pro," Redknapp told the club's official website.
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“He can run all day long, he’s good on the ball, a good passer, he sees all the pictures in front of him and is a top professional.
“We’ve got plenty of options in that department now, but he’ll certainly add to what we’ve got – he’s a good, good player and will be a fine acquisition for us.”
O'Neil made 24 Premier League appearances for Sam Allardyce's side last season and is eager to guide QPR back to the top flight.
“The challenge here is an exciting one," said O'Neil.
“Obviously I know Harry well and he was a big factor in me coming here. The promotion season we had under him at Portsmouth was fantastic.
“He’s played a massive role in my development as a player and now I’m back playing for him again and I want to repay him by helping this club get back into the Premier League.”