Although no financial terms were announced, Thomas signed for only a $500,000 guarantee, according to the Associated Press. He can earn another $2.6 million based on plate appearances and staying healthy.
"Frank was pretty direct that this is where he wanted to go," Oakland general manager Billy Beane said on a conference call with reporters. "He wanted to be in the right situation with the right club, so it wasn't really that difficult putting [a deal] together."
Thomas will be formally introduced as a member of the A's on Thursday in a 3 p.m. PT news conference at McAfee Coliseum.
Cast adrift by the World Series champion White Sox, to whom he could offer little help during another injury-curtailed 2005 campaign, Thomas has found a believer in his recovery in Beane.
Thomas hit it off during a Dallas encounter at the Winter Meetings with Beane, who spent most of this month monitoring his therapy and gradual return to baseball-related activities.
That included having Thomas come to Oakland early last week to be checked out by the A's doctor and trainers.
"He underwent a number of stress tests, very strenuous, and Frank felt great," Beane said. "He has a very good chance of being ready in April. He believes he'll be ready in Spring Training, we're just taking a conservative approach mentally."
Thomas' rehabilitation progress -- and his amazing lifetime on-base percentage -- convinced Beane this was a move worth making.
Although even before all the medical research, Beane had a sense this was a guy he wanted in manager Ken Macha's clubhouse.
"When we met in Dallas, he was, from a visual standpoint, in the physically best condition of his career," Beane recalled. "He looked fantastic. And his sense of determination was impressive to everyone in that room. Face-to-face, he's an impressive guy. Everyone walked away impressed.
"It's a great fit for this club. Frank Thomas is a presence."
That he is a presence in the right-handed batter's box is all the better. Last season, the A's top three home run threats (Eric Chavez, Mark Kotsay, Dan Johnson) were left-handed hitters.
"When Frank is healthy, he is a huge right-handed presence," Beane said. "And that's been a big issue for us the last couple of years."
Thomas thus becomes an older, more experienced update on Erubiel Durazo, whom the A's turned away after his own injury-limited tenure in Oakland.
Durazo and Scott Hatteberg, who also departed as a free agent, split the majority of DH duties last season. Durazo also backed up at first base, and Thomas now becomes the reserve there.
Various leg injuries have limited Thomas to a total of 108 games the last two seasons. A left-ankle stress fracture ended his 2004 season in July, and he didn't rejoin the White Sox until the end of May 2005.
Two months after his return, he again was sidelined by pain in the ankle, days later diagnosed as another fracture.
Thomas, a former tight end at Auburn University who often carries as much as 260 pounds on his 6-foot-5 frame, must constantly watch his weight, which can contribute to the stress on his legs.
Before becoming merely dangerous in 2001, when an arm injury limited him to 20 games, Thomas was downright monstrous.
Consider these numbers for his first 11 seasons: 344 homers, 1,183 RBIs and a .321 career average.
Despite adding to his totals only modestly since 2003, Thomas left the Windy City as the White Sox all-time leader in several offensive categories, including homers (448), RBIs (1,465) and runs (1,327).
And he has always worked his own threat to an advantage, with the patience and discipline to lay off when being pitched around. He has drawn 100-plus walks in 10 of his 12 seasons with 500-plus plate appearances, resulting in a lifetime on-base percentage of .427.
For comparison, last season only New York's Jason Giambi (.440) topped that figure in the AL.
Another full season would thrill the A's and reward Thomas. He can earn $1.4 million in bonuses by avoiding the DL with an injury related to his left foot ($325,000 each on May 1 and June 15, then $375,000 on July 15 and Aug. 15) and another $1.2 million by reaching 550 plate appearances, the AP reported.
"You could get carried away with the superlatives," Beane said. "Not just his power, but his selectivity at the plate. That type of approach can carry over to the other guys in the lineup."
Thomas has already cemented his role in Major League history as one of only 10 men with a .300 career batting average, 400 home runs, 1,000 RBIs, 1,000 runs and 1,000 walks.
Eight of those are in the Hall of Fame. No. 9 is a guy who does his damage across The Bay.