Los Angeles closed to within 2 1/2 games behind wild card-leading Boston, which lost to the Yankees on Saturday.
"We need to win and we need help. It's as simple as that," Los Angeles manager Mike Scioscia said.
Jerome Williams (4-0) gave up one run on five hits over 6 1/3 innings to get the win.
Jemile Weeks knocked in both runs for the Athletics, who have dropped four of six. Guillermo Moscoso (8-10) surrendered four runs -- three earned -- on five hits in seven full frames to absorb the loss.
"He's a really tough guy, only bad pitch was the Hunter home run," said Oakland manager Bob Melvin about Moscoso. "All in all, he really pitched well. We hit some balls hard but not much to show for it."
The Angels scratched out a run during their first at-bat. Erick Aybar reached on an error to begin the inning and eventually scored on Hunter's two-out single to right field.
Scott Sizemore, Eric Sogard and Weeks hit three consecutive singles to produce the tying run in the fifth.
Aybar bunted for a base hit with one out in the sixth. A groundout moved him to second and he scored the go-ahead run on Bobby Abreu's single to right. Hunter followed by driving his 23rd home run of the season over the wall in left-center.
Jordan Walden nailed down his 32nd save despite allowing an RBI single to Weeks in the ninth.
Angels first baseman Mark Trumbo left after the sixth inning with right ankle discomfort...Oakland was 2-for-11 with runners in scoring position.