The 24-year old, who spent the 2010 season with Toronto FC, was taken as the 12th overall pick in the draft by expansion franchise Vancouver Whitecaps who then traded him to Seattle for allocation money.
Sounders head coach Sigi Schmid was delighted with the acquirement of the Jamaica international.
"O'Brian is a forward with good pace and size," Schmid said. "He adds another option for us in our desire to be a club that scores goals."
Sounders general manager Adrian Hanauer added: "We had been looking at a number of players who were available in the Expansion Draft. O'Brian had a tremendous college career and he adds depth and competition at the forward position. He can strike from distance with both feet and is a player that can be impact on our roster."