Cust is likely to be the club's primary designated hitter. General manager Billy Beane has been committed to upgrading Oakland's offense.
"I'm happy he's back," manager Bob Geren said in a telephone interview. "He led the team in homers the last three years and walks. He's been big part of our offense. One thing we've lacked is home runs, which he can provide. It's a great match and everything's worked out perfectly on both ends."
Cust can earn an additional $350,000 in performance bonuses and would get the full amount if he has 600 plate appearances.
He became a free agent last month when the A's failed to offer a 2010 contract. He hit .240 with 25 homers and 70 RBIs last season. But he also struck out 195 times, becoming the first player to lead in the AL in three straight seasons since Minnesota's Bobby Darwin from 1972-74. Cust struck out 164 times in 2007 and 197 times in 2008.
He had 26 homers in 2007 and 33 the following year, and has topped Oakland for three consecutive seasons in homers, strikeouts and walks (93 last season).
Cust made $2.8 million last season and had been eligible for salary arbitration.
While Cust played 51 games in right field in 2009 and 96 as DH, he is more likely to focus on hitting only with the other offseason acquisitions made by Oakland. Defense is not his strong suit, though Geren said Cust has improved his outfield play and still could be an option there.
"What's exciting about Jack is he's becoming more of an all-around player," Geren said.
Oakland also was included in the big four-team, nine-player swap featuring Cliff Lee and Roy Halladay that brought outfielder Michael Taylor to the A's. Oakland sent third baseman Brett Wallace to Toronto.
Fox also can play the outfield and even catch.
"We have a lot of versatility which is good, especially with the way things happen over the course of the year," said Geren, whose teams have been plagued by injuries in recent seasons.
Last week, the A's brought back right-hander Justin Duchscherer on an incentive-laden one-year contract after he missed all the 2009 season. Duchscherer underwent arthroscopic right elbow surgery on March 31 and later was treated for clinical depression.
The A's finished with 75 wins for the second straight season and in last place in the AL West, losing their final seven games.