OAKLAND -- The A's, who publicly declared that they were in full-blown rebuilding mode after trading ace Dan Haren and popular outfielder Nick Swisher for prospects during a busy offseason, made it clear Thursday that they intend to capitalize on the young club's hot start and make a spirited run at the postseason.

After days of speculation surrounding Oakland's interest in free agent slugger Frank Thomas, who was released by Toronto on Sunday, the club signed the Big Hurt for the remainder of the 2008 season.

"I don't see this as an infringement on what we started in December when we traded Dan, but we have a responsibility to take advantage of opportunities like this," A's general manager Billy Beane said before the matinee finale of a three-game series against the visiting Twins at McAfee Stadium.

"Are we trying to develop a young team? Yeah, but we are not running an instructional league. We are running a professional sports franchise."

Thomas, who signed an incentive-laden, one-year deal with the A's in 2006 and led them to the American League West title and the AL Championship Series with team highs of 39 homers and 114 RBIs, turned down a two-year offer to return to Oakland that offseason.

He signed a two-year contact with the Blue Jays, who sweetened the deal by adding a $10 million option for 2009 that would have kicked in were he to make 369 plate appearances this season.

"I never wanted to leave here," Thomas said. "The finances didn't work out a couple of years ago, but I wanted to stay, sincerely. So when Toronto released me, this was my first choice, to come back here.

"It feels like deja vu. I'm happy and excited to be back."

Thomas, who led the Jays with 26 homers and 95 RBIs in 2007, was batting .167 (10-for-60) with three home runs and 11 RBIs in 16 games for the Jays when he was released. He said he has "no doubt" that his release was tied to his 2009 option and insisted that he has plenty left in his 39-year-old tank.

"This year wasn't a slow start. It was a huge start," said Thomas, who homered in three consecutive games (April 5-8) and had 11 RBIs in Toronto's first seven games this year. "I produced 11 runs in 16 games. I hit three balls out. I scored eight runs. I took 11 walks. That's production."

Whatever production Thomas can provide for the A's, who entered Thursday's game tied for the last among Major League teams with nine home runs, it'll come cheaply. Oakland must pay Thomas only a pro-rated portion of the Major League minimum of $390,000, while Toronto must absorb the rest of what's left on Thomas' reported 2008 salary of $8 million.

"He looks in great shape," Beane said during batting practice. "He had a great year for us. He's a great influence on the club. He's a consummate pro. This was an opportunity that could not be passed up."

The option clause does not carry over, said Thomas, who was in Oakland's starting lineup as the cleanup hitter and designated hitter.

"It didn't work out in Toronto, but no hard feelings," he said. "That's the business side of the game. I'm just happy to be back here. I feel comfortable here. ... Hopefully, I can get hot and do what I do."

The A's, who entered Thursday's game tied with the Angels atop the AL West standings, have been using left-handed-hitting Jack Cust and righty-swinging Mike Sweeney in something of a DH platoon for much of the early season.

Asked if Thomas was now his full-time DH, A's manager Bob Geren said, "Yeah."

Geren wouldn't say exactly what that will mean for Cust, Sweeney or any other player whose status might be affected by the addition of Thomas, but Sweeney started at first base Thursday, and he might end up splitting time there with rookie Daric Barton.

Cust, who was not in the starting line lineup, could end up in a platoon in left field with Emil Brown, a right-handed hitter who batted .317 against lefties in 2007 and .217 against righties.

"I talked to all the guys," Geren said. ""We're just trying to win as many games as we can. The emphasis has always been on winning. That's never changed."

To make room for Thomas on the 40-man roster, the A's transferred third baseman Eric Chavez from the 15-day disabled list to the 60-day DL. To create an opening on the 25-man roster, Oakland placed outfielder Travis Buck on the 15-day DL with shin splints.