It was the 2015 debut for O’Malley, who appeared in 11 games for the Los Angeles Angels last season, but spent all year in Triple-A before being called up Tuesday. He also had a run-scoring single in the second.
The Mariners had runners at second and third with no outs in the eighth inning after an error by Pat Neshek (3-5). The miscue came when he fielded a sacrifice bunt by Jesus Sucre but his throw sailed over the head of 5-foot-5 Jose Altuve, who was covering first base.
O’Malley’s line drive to right field scored Logan Morrison to put Seattle on top 4-3.
Morrison, who hit a game-winning two-run homer as a pinch-hitter on Tuesday, connected off Josh Fields to make it 8-3 in the ninth.
Seattle starter Taijuan Walker allowed five hits and three runs in 6 2-3 innings. Carson Smith (2-5) got the last out of the seventh inning for the win and Tom Wilhelmsen got the final five outs for his eighth save.
Franklin Gutierrez had two hits and an RBI for the Mariners, who took two of three from the AL West-leading Astros.
Scott Kazmir yielded seven hits and three runs — two earned — in 4 1-3 innings, which was his shortest start since a trade to Houston on June 23.
The Mariners had a shot to take the lead in the seventh when they loaded the bases with one out. But they came away empty when Colby Rasmus caught a long fly ball to right field by Mark Trumbo and his throw beat Nelson Cruz home to finish the double play. Cruz didn’t return after that play because of a strained right quadriceps.
Seager doubled to start the fifth and an RBI single by Robinson Cano with one out chased Kazmir and tied it 3-3.
The Mariners tied it at 2-all in the third on a ground-rule RBI double by Gutierrez, which bounced on the hill in center field and over the wall.
A two-out RBI single by Evan Gattis put Houston on top 3-2 in the bottom of the inning.
Preston Tucker, who was recalled from Triple-A Fresno earlier Wednesday, hit an RBI single tie it 1-1 in the second. The Astros took a 2-1 lead on a sacrifice fly by Luis Valbuena later in the inning.
Astros: An MRI on the right shoulder of RHP Scott Feldman revealed no damage and he received a cortisone injection to calm the inflammation in his shoulder. Feldman left Tuesday’s start after 2 2-3 innings with discomfort in his throwing shoulder. The Astros hope he will be OK after sitting out one start. … OF George Springer (broken right wrist) will play his fourth rehabilitation game at Double-A Corpus Christi on Thursday and could be activated soon after.
Mariners: Seattle is off Thursday and opens a three-game series at Oakland on Friday when LHP Edgar Olmos (1-0, 2.13) opposes A’s RHP Aaron Brooks (1-1, 5.47). It will be the second career start for Olmos, who has appeared in five games with one start for the Mariners this season.
Astros: Houston has a day off Thursday before RHP Collin McHugh (14-7, 3.92) opposes Minnesota RHP Mike Pelfrey (6-8, 3.85) on Friday in the opener of a three-game series. McHugh is second on the team behind ace Dallas Keuchel with a career-high 14 wins.