San Diego got Pomeranz, minor league lefty Jose Torres and either a player to be named or cash from Oakland on Wednesday for Alonso and reliever Marc Rzepczynski, who reached agreement on a one-year deal to avoid arbitration.
Pomeranz can either start or come out of the bullpen. He had surgery on his pitching shoulder in October and is expected to be ready by spring training. He was 5-6 with three saves and a 3.66 ERA in a career-high 53 appearances with nine starts spanning 86 innings for the A's in 2015.
Padres general manager A.J. Preller said Pomeranz "gives us the option for either" starting or relieving. He said he'll discuss Pomeranz's role with pitching coach Darren Balsley and manager Andy Green, and make a call "as we get closer to spring."
Pomeranz went 10-10 with a 3.08 ERA in 73 games, including 19 starts, in two seasons with the A's after he was acquired from Colorado following the 2013 season.
It was a busy day for both California clubs, with San Diego naming Mark McGwire bench coach and the A's agreeing on a $2 million, one-year contract with lefty Felix Doubront -- a deal that includes $500,000 in guaranteed money and avoids salary arbitration. The A's also reached deals with infielder Eric Sogard and outfielders Sam Fuld and Andrew Lambo.
The A's had been investigating their options at first base through the trade market and free agency.
"Yonder was a guy we asked San Diego about right after the season. One of, if not the best, defensive first basemen in the game," general manager David Forst said. "Obviously the knock on him is he doesn't profile the power of a first baseman but still a very productive offensive player, high contact rate, low walk-to-strikeout rate, a guy who's a professional hitter. Excited to have him under control for a couple years."
Torres pitched for Oakland's Class A affiliates at Beloit and Stockton last year, and combined for a 4-5 record, eight saves and a 2.56 ERA in 47 relief appearances. He struck out 84 batters and surrendered just four home runs in 77 1/3 innings while holding the opposition to a .203 batting average. The 22-year-old left-hander was originally signed by the A's in July 2010 as an undrafted free agent out of Venezuela and pitched exclusively in relief for the first time in 2015.
Rzepczynski fills a need for the A's in their bullpen. He began the 2015 season with Cleveland and was 2-3 with a 4.43 ERA in 45 games when he was traded to San Diego on July 31. He compiled a 7.36 ERA in 27 games with the Padres and combined for a 5.66 ERA in 72 games overall.
"Adding Marc certainly softens the blow of losing Drew in the bullpen," Forst said.
Alonso's four seasons in San Diego were mostly marked by injuries. He played in 155 games in 2012 after being obtained from Cincinnati in a five-player deal but has been limited to 97, 84 and 103 games due to various injuries.
Alonso batted.282 with five home runs and 31 RBIs in 2015. He was on the disabled list from May 9 to June 1 with a bone bruise in his right shoulder and Sept. 14 through the end of the season with a lower back strain.
For Alonso, the vote of confidence from Oakland is appreciated.
"It means a lot; I talked to (manager) Bob Melvin earlier today. I'm thrilled to have a group of guys and organization who think highly of my defensive skills," he said.
Wil Myers is expected to take over at first base for the Padres.