Stefan Romero's three-run homer off Evan Scribner gave Seattle a walk-off victory.
"I was just looking for something up to drive to get the sac fly," said Romero, who has never played above Triple-A. "Luckily, it went out."
Romero can play both corner outfield positions and has a real chance at making the big league roster when camp breaks this week. Manager Lloyd McClendon has taken notice.
"He's been playing well for about, say 2 1/2 weeks," he said. "Been swinging the bat."
Dustin Ackley hit a two-run homer off Oakland starter Josh Lindblom in the fifth. Justin Smoak and Logan Morrison each had two hits for the Mariners, and Kyle Seager had an RBI double in the ninth.
Josh Donaldson and Derek Norris homered for the A's. Donaldson, who finished fourth last year in American League MVP voting, crushed a two-run shot off Mariners starter Randy Wolf in the fourth inning for his third homer of the spring.
Norris hit a solo shot off Wolf in the sixth and Cespedes added a solo homer off Yoervis Medina in the eighth that gave Oakland a 4-2 lead.
A's manager Bob Melvin was pleased to see cleanup hitter Cespedes, who finished 2-for-3 with a walk, go deep.
"When you're a power hitter, it's always nice to get that first homer out of the way," he said. "But his at-bats were better today. Couple of hits, a walk, a homer."
GRAY FOR OPENING DAY
Before the game, Melvin announced that Sonny Gray would be the A's opening day starter against Cleveland.
During his rookie season last year, Gray, a 24-year-old right-hander, started 10 games, posting a record of 5-3 with a 2.67 ERA and striking out 67 batters in 64 innings.
Gray will be followed in the rotation by Scott Kazmir, Jesse Chavez, Dan Strailey and Tommy Milone.
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