"I'm very grateful for the opportunity given to me by Vancouver Whitecaps FC," O'Brien told the Whitecaps' official website.
"I appreciate the hard work that's been put in by Vancouver, given the short period of time there was to get this deal done.
"I've had the opportunity to look around the city, which is fantastic, and I'm really looking forward to contributing on and off the pitch."
Whitecaps head coach Martin Rennie said: "Andy is an experienced centre-half, who is good in the air and comfortable on the ball.
"He has spent several years in the English Premier League and with some of England's best-known clubs. Andy also had the privilege of being at a World Cup.
"We're delighted to add a defender of Andy's experience to our squad."