The other homer? A grand slam from Brandon Moss, the first of his career.
"I would think he's hit about 10 of them -- this year," said manager Bob Melvin.
"No, I tell guys all the time, I've never come close to hitting one," said Moss, who clobbered a 96-mph fastball from Garrett Richards. "It's definitely something I had thought about. I hit fastballs really well, and right there you know he's going to throw you one. It was on my mind. I definitely wasn't trying to do it, but thought it would be cool to."
Moss clocked out early with a mild left calf strain in the third inning, but he had seemingly put in a full day's work already.
The slugger's 13th homer highlighted a five-run first inning against Richards, who also walked three and totaled just two outs before getting the hook.
"It's embarrassing," said Richards. "I didn't show up for us tonight, and I didn't give us a chance to win, and I put us in a hole early."
Lefty Wade LeBlanc didn't fare much better, though, as the A's improved to 21-3 when scoring first.
The Angels southpaw was done in by Donaldson, who tagged him for an opposite-field homer in the second and a two-run laser shot to dead center in the fifth. In between, the third baseman collected a third-inning RBI single, finishing the night with four RBIs.
It marked the second career multi-homer game for Donaldson, who has a team-high 15 on the season, after hitting 24 last year. His 45 RBIs are second to Moss' 46.
That's 91 RBIs between the two -- and 29 home runs, just one-third of the way through the season.
"When Moss is swinging the bat like that, JD's going to get some good pitches to hit," said Melvin. "And until recently, [Yoenis] Cespedes had been driving in runs and swinging the bat well too, and those three guys in the middle of the order kind of insulate Moss between two righties and protect each other."
"I've been seeing some pretty decent pitching for a while now," said Donaldson. "It's just a matter of squaring it up or not. And Moss has had a great year. It's kind of one of those ordeals where you have to pick your poison I feel like."
The A's had dropped six of their previous eight, but are still 14-7 over the last 21 and 33-22 overall, good for the second best record in the AL.
Lefty Drew Pomeranz gave them a season-high 5 1/3 innings, but also surrendered five runs on only four hits, with the slugfest extending to the Angels' side, too.
Mike Trout fanned twice against Pomeranz, but also homered to lead off the fourth and put the Angels on the board. Two batters later, Howie Kendrick took the lefty deep for a two-run shot.
Pomeranz, who had allowed just one home run in his previous 24 innings, retired six of his next seven before boarding a pair of runners with one out in the sixth, forcing Melvin to go to righty Dan Otero.
Otero proceeded to allow both runs to score on C.J. Cron's ensuing double, leaving Pomeranz with a season-high five runs to his name. He had yielded five total in 32 2/3 innings entering the night. The lefty walked three and struck out five in the outing.
"I did what I wanted to accomplish early, attacking guys, getting ahead of guys," said Pomeranz. "I made one bad pitch to Trout. I didn't want to throw it there. The other home run, he just got me. Of course the walks don't help. I may just be trying to do too much at that point to make sure I stay in the game, make sure I'm being effective and efficient."
Pomeranz said he thanked Donaldson "about 10 times" for the assist. Otero, Luke Gregerson and Sean Doolittle also helped out with 3 2/3 scoreless innings.
"Josh is playing really well right now across the board," said Melvin. "He's got great focus, he has a great idea how he's gonna get pitched on a particular night, and his defense always comes with him, whether or not he's swinging the bat well or not. It was a good night for him all the way around."