OAKLAND, Calif. -- It was the bottom of the first inning, and the Seattle Mariners were already down 4-0 after Oakland A's third baseman Danny Valencia hit his third career grand slam Friday night at the O.co Coliseum.

Early in the season, when their bats were silent, the Mariners' hopes of digging out of a big early hole were slim.

And now?

Left fielder Stefen Romero and third baseman Kyle Seager hit two-run homers, and the Mariners roared back for an 11-8 victory.

"There's so much more confidence, especially as of late," Seager said. "And it's been like this for a little while. The offense has been really swinging it like we thought we should have been the whole year."

Oakland led 5-1 after two innings, but the Mariners scored six runs in the third and two more in the fourth on Romero's first home run of the season, giving them a 9-5 lead. Romero came off the bench after the top of the second, replacing Franklin Gutierrez, who left with a groin injury.

Romero was recalled Tuesday after playing 116 games for Triple-A Tacoma. He had an RBI single in the third then sent Fernando Abad's first-pitch fastball over the right-center-field wall after Seager lined a leadoff single.

"Coming up here, I just didn't want to change my approach from when I was down in Triple-A and so far I feel like I've had good at-bats and my approach has been solid," Romero said.

With the Mariners clinging to a 9-8 lead in the ninth, Seager hammered a two-run shot into the right-field seats off left-hander Drew Pomeranz after shortstop Ketel Marte hit a leadoff single.

Seager went 3-for-4 with a double and home run, drove in three and scored three times. Romero was 2-for-4 with three RBIs and scored twice. First baseman Logan Morrison went 3-for-4 with a double, drove in two runs and scored once.

"A lot of fight, a lot of good at-bats," Seager said.

Second baseman Brett Lawrie went 3-for-5 with an RBI double and scored a run for the A's. Valencia went 2-for-5 and drove in four runs.

"They've been doing it for a while now, power wise and productive at-bats," A's manager Bob Melvin said of Valencia and Lawrie.

Oakland scored three runs in the sixth off right-hander Mayckol Guaipe, cutting Seattle's lead to 9-8. Designated hitter Billy Butler hit a leadoff double and moved to third on pinch hitter Coco Crisp's infield single. Shortstop Marcus Semien drove them in with a triple to right-center field and scored on center fielder Billy Burns' infield single.

The A's had runners on first and third with two outs in the seventh, but right-hander Carson Smith retired Smith on a foul line drive to Morrison.

Smith pitched a scoreless eighth, striking out Valencia with two outs and two runners on base. Tom Wilhelmsen pitched a scoreless ninth for his ninth save.

Mariners left-hander Edgar Olmos lasted only 1 1/3 innings in his second major-league start, giving up five runs (four earned) and six hits but got a no-decision.

"He didn't have much tonight," Mariners manager Lloyd McClendon said of Olmos.

A's right-hander Aaron Brooks (1-2) gave up six runs and six hits over 2 1/3 innings in his fifth start since coming to the A's from Kansas City in a trade on July 28.

"It was a bad night to have a bad night, especially after what our offense did," Brooks said. "I feel like I took the whole loss."

Mariners right-hander Jose Ramirez (1-0) blanked the A's for 1 2/3 innings after relieving Olmos and got the win.

Burns lined a leadoff single in the first, Canha reached on a fielding error by Seager, and Lawrie singled to left, loading the bases for Valencia. Valencia launched Olmos' 1-1 fastball over the 362-foot sign in right-center field, going the opposite way.

"It's always nice to get off to a good start, but it wasn't enough," Valencia said.

Seattle cut Oakland's lead to 4-1 with a run in the top of the second. Right fielder Seth Smith walked, moved to second on Morrison's one-out single and scored on center fielder Brad Miller's sacrifice fly.

Oakland made it 5-1 in the bottom of the second. Canha ripped a one-out single, and Lawrie brought him home, lining a double to deep center field.

Seattle answered with six runs in the third, taking a 7-5 lead. The Mariners sent 10 batters to the plate and had seven hits, including doubles by Marte, Seager, Morrison and Miller.

NOTES: Mariners LHP Joe Beimel (left shoulder inflammation) was activated from the 15-day disabled list. ... Mariners DH Nelson Cruz (strained right quad), who left Tuesday's game against Houston, was out of the lineup against Oakland. ... Mariners LHP James Paxton, who has been on the disabled list with a strained left middle finger, tore his nail on that finger Thursday night in what had been expected to be his final rehab start before rejoining the rotation. Paxton won't resume throwing until Monday at the earliest. ... Oakland LHP Sean Nolin was recalled from Triple-A Nashville on Friday and will start Sunday against Seattle in place of injured RHP Chris Bassitt (sore right shoulder). ... A's C/1B Stephen Vogt (sore right elbow), who was slated to start at first base, was a late scratch from the lineup but pinch hit in the seventh and stayed in the game at catcher.