Thomas, who lost to eventual champion Lance Niekro of the Giants in the first round of last year's inaugural event, came back to beat A's teammate Esteban Loaiza, 13-12, on a last-second home run.
"It came down to the wire," Thomas said. "It's more fun when it's like that. I think most of the games all night were close."
In addition to being for a good cause, Thomas said the event also promotes goodwill between the Bay Area rivals.
"It switches things up," he said. "You hang with guys who you play against. You see them in a different way."
The format was a home run derby, with players able to choose anyone in the Major Leagues. Thomas went with his good friend Andruw Jones of the Atlanta Braves. Loaiza used Alfonso Soriano.
"It was tied at 12 with a second left," Loaiza said. "Just as he hit, my pitcher was in the windup and then the [timed] game stopped. At least I hit one 532 feet. I don't think [Thomas] got over 350."
With "Extra Innings" host Robert Baum serving as emcee, the event drew a good crowd of fans.
"It's fun," said Harden, who bowed out in the first round using Ryan Howard. "You can do all the practicing you want, but it's different once you get up to the big screen. I was impressed that pretty much all of the A's showed up. It was a good turnout."
Baum, who nearly didn't get the chance to reprise his role as host, drew rave reviews, too.
"We definitely wanted him to be a part of this," Harden said. "The fans know him, he's a good guy and good at what he does."
Anyone interested in getting involved can still do so through the Web site.
Back at the camp:
Shortstop Bobby Crosby is one step closer to playing in his first spring game after participating in a simulated game on the back field at Phoenix Municipal Stadium.
"He was awesome," A's manager Bob Geren said. "He might have had a whole week's worth of action. He's further along than most players who are facing pitchers for the first time."
Jeff Gray, who ended the season at Class A Stockton last year, pitched to Crosby, who had nine at-bats. Among his more memorable shots were a home run that landed close to Van Buren Ave., the street just beyond the left-field fence, and a shot off the top of the fence. He was also hit by a pitch in his first at-bat.
"That's as close to a game as you can get," Geren said. "He was happy about it because he had something to prove to himself."
Crosby was hopeful of repeating the routine again on Tuesday at the Minor League camp in preparation for facing Loaiza in a simulated game on Wednesday.
Erubiel Durazo, who is trying to make the team as a non-roster invitee, provided the most impressive swing of the day in a game against the Mariners. He hit the first pitch he saw from left-hander George Sherrill over the right-center-field fence, the deepest part of the stadium, for a three-run home run.
What makes the drive even more impressive is the fact that Sherrill did not allow a home run in 72 appearances for the Mariners last year and left-handers hit .143 against him.
"He's playing like a big-league player," Geren said. "He's the most diligent of anybody in getting his work in this spring. It's going to be tough, and I hope we have a real tough call."
The A's optioned right-hander Shane Komine and outfielder Javier Herrera to Triple-A Sacramento and reassigned lefty Derek Thompson and catcher John Baker to their Minor League camp.
Right-hander Dan Haren (0-0, 3.60 ERA) makes his third start of the spring when the A's travel to Scottsdale for a Tuesday night game. He'll be opposed by Giants lefty Noah Lowry, Haren's college teammate at Pepperdine.