Pomeranz (1-1) retired David Freese on a grounder to second base and Raul Ibanez on a foul pop to the catcher before Iannetta drove the next pitch to center field and just out of the reach of a leaping Craig Gentry, ending Oakland's four-game winning streak.
Angels manager Mike Scioscia got six scoreless innings out of his much-maligned bullpen, as the Athletics stranded runners in scoring position in the 10th, 11th and 12th. Joe Smith (1-0), who failed to retire any of the five batters he faced in the eighth inning of the Angels' 10-9 loss on Tuesday night, pitched a scoreless inning for the victory.
Brandon Moss hit a three-run homer for the A's in the fourth inning.
Oakland's Jed Lowrie poked an opposite-field double inside the third base bag with one out in the 10th, but Michael Kohn struck out Yoenis Cespedes after Lowrie advanced on Josh Donaldson's flyout to center field.
Oakland left-hander Tommy Milone was charged with three runs - two earned - and six hits over six-plus innings in his second start and departed with a 4-1 lead. Los Angeles closed to 4-3 in the seventh with a pair of runs after a costly throwing error by first baseman and former Angel Alberto Callaspo, who had never played that position in the majors or the minors until this season.
Dan Otero relieved Milone with two on and none out and Erick Aybar grounded into a force at second. J.B. Shuck followed with a grounder to Callaspo, who tried to start an inning-ending 3-6-3 double play. But his hurried throw into the runner couldn't be handled by shortstop Nick Punto, and Iannetta scored from third. Albert Pujols added an RBI single one out later.
Callaspo also dropped a relay throw from second baseman Eric Sogard on a potential double-play grounder in the sixth by Freese. But the next batter, Ibanez, grounded into a 4-6-3 DP to end the inning.
The A's, who beat the Angels 10-9 Tuesday night without the benefit of a home run, snapped a 1-1 tie in the fourth against Tyler Skaggs when Moss drove a 2-1 pitch into the right-field seats with two outs for a three-run homer after Callaspo's RBI double. Skaggs was charged with four runs, seven hits and three walks in six innings.
NOTES: The Angels will honor the late Jim Fregosi during a two-game interleague series against Philadelphia Aug. 12-13. Fregosi, who died Feb. 14 at age 71, was a six-time All-Star who spent his first 11 big league seasons as an Angels SS and managed them to their first division title in 1979. He also piloted the Phillies to a pennant in 1993. ... This was the first time the Angels played four extra-inning games on the same homestand since June 1975. ... Pinch-hitter Coco Crisp, who didn't start in any of the three games because of tightness in his left hamstring, drew a leadoff walk in the 11th and stole second one out later to pass Reggie Jackson for fifth place in Oakland history at 145. ... The A's claimed RHP Marcus Walden off waivers from Toronto and optioned him to Triple-A Sacramento. ... Pujols nearly took out Skaggs when he pulled a pitch into the Angels' dugout right after Trout's RBI double. ... The Angels begin a nine-game road trip Friday night at Detroit. Jered Weaver will make his fourth attempt at his first victory of the season, while trying to extend the Halos' winning streak against the Tigers to 10 games.