Callaspo hit a tiebreaking two-run double in Oakland's four-run ninth inning, leading the Athletics to a 5-1 victory over the Toronto Blue Jays on Monday.
Callaspo, who was acquired in a trade with the Los Angeles Angels on July 30, also had a tiebreaking double in the eighth inning of Sunday's 6-4 win at Toronto. The clutch hit in the series finale came against closer Casey Janssen (4-1) with one out and the bases loaded.
"It gets you closer to your team," Oakland manager Bob Melvin said of Callaspo's pair of big swings. "It's very difficult to come into a club late in the year and feel comfortable. The more you do that in those type of situations, the more comfortable you feel, the more part of it you feel, and rightly so."
Callaspo is batting .292 with four RBIs in 10 games with Oakland. Teammate Josh Donaldson said he's happy to have Callaspo on his side.
"It shows that the trade paid off," Donaldson said. "He's a solid player, he's scrappy. Against us it seemed like he would get that big RBI to either put them back in the game or give them the lead. So far, he's been doing the same thing over here."
Monday's decisive hit came after Callaspo made an error in the eighth that allowed the Blue Jays to score the tying run.
"He definitely made up for it," Melvin said.
The Athletics had lost six of seven before arriving in Toronto, but took three of four from the last-place Blue Jays. Oakland began the day one game back of AL West-leading Texas.
"I feel like it definitely got us back on the right track," Athletics starter Dan Straily said.
Oakland outfielder Josh Reddick, who hit five home runs in the first two games of the series, failed to go deep for the second straight game. Reddick walked in the second, flied out in his next two at-bats and was intentionally walked in the ninth.
Donaldson singled off Janssen to begin the ninth and, one out later, pinch hitter Brandon Moss doubled him to third. Reddick was walked to bring up Callaspo, who sent a 1-0 pitch into the right-field corner.
"I was glad to get that chance and then contribute for the team," Callaspo said. "I feel happy right now."
"I gave up that leadoff single and it just kind of snowballed on me," Janssen said.
Oakland got a big boost from Straily, who had lost four consecutive starts. The rookie allowed one run and six hits in a career-high 7 1/3 innings.
"He was dominant out there," Donaldson said. "We needed to get this series victory and he showed up today and was able to get the job done."
Frustrated by Straily through the first seven innings, the Blue Jays tied it in the eighth. Jose Reyes hit a one-out single and was running when Maicer Izturis followed with a ground-ball single that got through because shortstop Jed Lowrie had broken to cover second, allowing Reyes to reach third.
Cook replaced Straily and Reyes came home when third baseman Callaspo couldn't handle Jose Bautista's sharp grounder. Edwin Encarnacion followed with a liner right at second baseman Sogard, who doubled off Izturis to end the rally.
Happ allowed three hits in a season-high seven innings. He set down 14 straight batters between the third and seventh innings.
Pitching just days after the death of his grandfather, Happ struggled to contain his emotions as he spoke with reporters in the clubhouse.
"I definitely had a heavy heart," he said. "I tried to use it."
The left-hander was moved up a day to replace Josh Johnson, who was pushed back to Wednesday with a sore forearm. The Blue Jays intend to recall right-hander Todd Redmond from Triple-A Buffalo to start against Boston on Tuesday. Happ was placed on the three-day bereavement list to open a roster spot for Redmond.
Oakland OF Coco Crisp (left wrist) and C Derek Norris (back) were held out of the starting lineup. ... Toronto OF Colby Rasmus (abdominal strain) was not available. ... The Blue Jays have lost nine of 13 at home. ... After playing four straight games on artificial turf, Oakland OF Yoenis Cespedes is expected to DH when the Athletics return home Tuesday to host Houston. ... Oakland INF Adam Rosales, who was designated for assignment Saturday, has been claimed on waivers by Texas. ... With a day off in Toronto before their three-game series with the Blue Jays begins Tuesday, Red Sox players Jonny Gomes, Dustin Pedroia and Jarrod Saltalamacchia attended the game.