Cust homered and doubled in the go-ahead run during a six-run eighth inning, and Oakland rallied from a seven-run deficit to beat the Angels 15-10 on Saturday night.
"You don't want to mess with a team that's out of it like they are," Los Angeles center fielder Torii Hunter said. "A lot of guys over there are fighting for positions next year, and they come hungry. I'm not saying we don't, but they're starving. They're like hyenas right now. They're playing great baseball."
The Angels, whose magic number to clinch their third straight AL West title remained at four, lost their season-high fourth straight. They were the only team in the majors that hadn't lost more than three in a row this season.
The Angels lead Texas by five games with eight to play, including a four-game set against the Rangers that begins Monday night at Angel Stadium.
"We've just got to get to our first goal. It doesn't matter when it happens, really," Angels pitcher John Lackey said.
The A's have won 11 of their last 13, a stretch that began with a three-game sweep at Texas, and they can sweep the Angels with a victory Sunday.
"It's a huge confidence booster," Oakland's Ryan Sweeney said. "To be down 9-2 in the fourth and come back and win, it's a huge win for us. We've been putting it together pretty good over the last couple of weeks. It's a little bit too late, but we've got nothing to lose right now. So we've just got to go out and try to improve and put together as many good ballgames as we can."
Kurt Suzuki and Mark Ellis also homered for the A's.
Rookie Kevin Jepsen (6-4) was charged with four runs and three hits in the Oakland eighth, including a tying RBI single by Suzuki after the right-hander struck out Rajai Davis and Sweeney with runners at the corners. Cust's double gave the A's an 11-10 lead, and Daric Barton greeted Matt Palmer with an RBI single before Ellis capped the rally with a three-run homer.
Craig Breslow (8-7) pitched 1 1-3 innings for the victory after giving up an RBI single by pinch-hitter Gary Matthews Jr. that put the Angels ahead 10-9 in the seventh.
"Tonight they hit the ball hard. They had 19 hits and there were no cheapies," Hunter said. "They beat us. If we're going to get beat, we should get beat like that."
Cust was 3 for 5 with four RBI and Barton was 4 for 5 with two RBI. Suzuki had three hits and a season-high four RBI during Oakland's 19-hit outburst, raising his team-high RBI total to 85.
"Kurt's doing an unbelievable job," Sweeney said. "He catches every single day. And to have that grind on your body and still produce the way he does is pretty impressive."
Darren Oliver relieved Lackey after five innings with a 9-6 lead and gave up two runs in the sixth. Adam Kennedy hit an RBI double and scored on a two-out wild pitch to Sweeney.
Lackey allowed six runs and 10 hits. The right-hander, who hadn't allowed a home run in his previous five starts, gave up a two-run shot by Suzuki in the first and a three-run shot by Cust in the fifth.
"That last inning I kind of got caught in a situation where I didn't want to walk anybody and threw a couple of 3-2 heaters pretty much down the middle," Lackey said. "Sometimes a team with nothing to lose is pretty dangerous. The offense was there for us today, and I stunk."
Suzuki gave the A's 2-0 lead in the first with his 15th homer after a two-out walk to Sweeney. Juan Rivera responded in the bottom half with a three-run shot over both bullpens, giving him new career highs in homers (24) and RBI (86).
All three runs off Dana Eveland in the Angels' first were unearned after shortstop Cliff Pennington botched a potential double-play grounder by Bobby Abreu when he took a throw from second baseman Ellis off the bag and wasn't able to get the out at first, either.
The Angels increased their margin to 9-2 with six runs in the fourth. Jeff Mathis hit an RBI single, and Kendrick scored when Kennedy hit him in the back with his throw to the plate after fielding Maicer Izturis' grounder. Chone Figgins chased Eveland with a two-run single and Abreu hit a two-run homer on Brad Kilby's first pitch, giving him 100 RBI for the season.