Frieri (0-1) was trying to preserve a 2-1 lead for starter Hector Santiago when Josh Donaldson led off the ninth with a single and Cespedes flied out. Jaso batted for Derek Norris and drove a 1-2 pitch deep into the right-field seats for his first homer of the season and the first by an A's pinch-hitter.
"Donaldson hit that ball hard to start off the inning, and it kind of started there," Jaso said. "I think Cespy just missed a pitch, too, so we definitely had some good swings going and some good momentum against Frieri.
"He showed me one changeup earlier in the at-bat and got a strike, so I thought he was going to go back with an off-speed pitch -- especially after I fouled off a couple of his fastballs," Jaso added. "But he came back with another heater and left it out over the plate. My timing was a little bit better on that particular pitch, and it looked really good to hit."
Jaso has six homers and 21 RBIs at Angel Stadium along with a .424 average, the highest by any player with at least 75 plate appearances at the "Big A."
"He got a really good pitch to hit and the ball went out," Frieri said. "In that situation, he's looking for something middle-in to drive in the air -- especially when they have a man on base. He hit a mistake pitch. Don't get me wrong, he made good contact, but I was trying to go down and away."
It appeared Frieri needed more consoling from Santiago than vice versa.
"I closed in 2012, so I know how it is, man. You've got to have a short memory and forget about it," Santiago said. "He's going to close more games than he blows. He said `Sorry' to me, but I told him: `No, don't worry about it, man. We're all out here battling.' I could have given up a two-run homer and given up the lead in the seventh."
Santiago allowed a run and five hits while striking out three in his fourth start for the Angels after spending his three previous big league seasons with the Chicago White Sox. A two-out homer by Cespedes in the fourth was Oakland's only run against the left-hander.
The Angels lost a replay challenge in the ninth after Howie Kendrick was ruled out by first base umpire Chris Segal on a grounder in the hole that second baseman Nick Punto bobbled before throwing to Daric Barton.
Jesse Chavez allowed two runs -- one earned -- and four hits in seven innings with nine strikeouts and no walks. Oakland's starting pitchers have yielded three earned runs or fewer in every game so far, the second-longest streak by an A's rotation at the beginning of a season since a 16-game stretch in 1981, when manager Billy Martin's starters were Mike Norris, Rick Langford, Matt Keough, Steve McCatty and Brian Kingman.
"It certainly hurts to lose two important guys like (Jarrod) Parker and (A.J.) Griffin," manager Bob Melvin said. "But when you have guys like Jesse Chavez and Tommy Milone that are going to step in and hold the fort down and compete at a high level, that's what we expect from those guys. So we have the depth."
Pujols, closing in on 500 homers, a celebrated milestone achieved by 25 players in major league history, has four homers and eight RBIs in his last six games after driving in only one run in 30 at-bats over his first seven games.
The three-time NL MVP also had an RBI single in the opener of a 19-game season series between the AL West rivals, who have combined to win 11 division titles over the past 14 years. The two-time Gold Glove first baseman frustrated the A's in the eighth by starting a slick double play on Jed Lowrie's grounder.
Oakland CF and leadoff hitter Coco Crisp didn't play because of tightness in his left hamstring. ... Los Angeles 3B David Freese was 0-for-4 with two strikeouts. He has five hits and 13 K's in his first 43 at-bats with the Angels.
Copyright by STATS LLC and The Associated Press
ANAHEIM, CA - APRIL 14: Jesse Chavez #60 of the Oakland Athletics throws a pitch against the Los Angeles Angels...
37,120 (81.6% full) - % is based on regular season capacity
78 degrees, clear
Home Plate - Tim Welke, First Base - Chris Segal, Second Base - Tim Timmons, Third Base - Todd Tichenor
Albert Pujols has hit .211 after falling behind in the count 0-1 this season, well below his career average of .301 in those situations. Jesse Chavez has thrown a first-pitch strike to 69 percent of batters this year, the eighth-highest percentage among qualified pitchers.