Zito, 37, pitched seven seasons for the A's from 2000-06, going 102-63 with a 3.53 ERA and winning the AL Cy Young Award after a dazzling 23-5 campaign in 2002.
Although he made no official announcement, Zito believed his playing career had concluded with the end of the minor league season earlier this month.
That's why he didn't initially know how to react when Athletics general manager Billy Beane called Monday and asked if he was up for pitching in the major leagues again.
"I turned the page on the game eight days before, so it was a little daunting and strange to have to pull that page back again after 15 years," Zito said Wednesday after joining the A's before the game with the White Sox.
"Just, was it the best thing for me? It ended up that my wife was really telling me all the things I needed to hear and ended up saying this is going to be a lot of fun," he said.
Zito didn't pitch in 2014 and attempted to make Oakland's major league club during spring training but was assigned to Nashville. He has started 22 games for the Sounds this season, going 8-7 with a 3.46 ERA and 91 strikeouts in 138 innings pitched.
Assistant general manager David Forst told the San Francisco Chronicle on Wednesday that Zito was called up strictly to pitch in relief and would not make any starts for the A's.
He last pitched for the A's in Game 1 of the 2006 American League Championship Series against the Detroit Tigers. After the '06 season, Zito signed a seven-year, $126 million contract with the San Francisco Giants. He hasn't pitched in the majors since 2013, his last season with Giants.
It is not certain, but Zito could pitch against former A's teammate Tim Hudson when Oakland hosts the Giants on Sept. 26. They were part of Oakland's Big Three along with Mark Mulder. Hudson is retiring after this season and Zito might follow.
"I'm really happy for him," Hudson said. "I'm sure he appreciates it and I think he deserves it. I'm sure the fans in Oakland will ultimately appreciate it as well."
"It's gonna be great to bring (Zito) home and get him in games in our place in front of our crowd," Oakland manager Bob Melvin said. "They love him there. Particularly when we play the Giants, it's gonna be a really exciting weekend. It's really gonna add to that having him there."
Zito's best days as a major leaguer were in Oakland, where he was a three-time All-Star selection. According to ESPN Stats & Information, Zito has the franchise's second-highest strikeout-per-batter rate in franchise history -- at just under 1 strikeout for every 5 batters (.183) -- since the A's moved to Oakland in 1968. Only Hall of Fame reliever Rollie Fingers has a higher rate, at .190 per batter.
With the Giants, Zito led the league in losses in 2008, never won more than 15 games in a season, and saw his career ERA rise by more than a run.
Zito said he hasn't thought about playing beyond the three weeks remaining in the season, but conceded it would be fitting if he ended his career in an A's uniform.
"Absolutely," he said. "This is where I started. That mound in Oakland is where I threw my first major league pitch, and I don't know how it's all going to shake out with the rotation, days and all that, but I'm going to throw one of my last major league pitches probably on that mound. That's like storybook. It's amazing."
The Associated Press contributed to this report.