With one out and the score tied 3-3 in the eighth inning, Nick Noonan singled off Reds reliever Pedro Villareal. Oropesa then cleared the right-center field barrier with his first spring home run. Perez also recorded his first Cactus League homer by launching a drive to left field.
Kensuke Tanaka and Francisco Peguero singled before racing home on Monell's double. Monell scored on Ehire Adrianza's single.
The Giants, who have won three of their last four games, snapped Cincinnati's three-game winning streak.
Both starters recorded their longest outings of the spring. Cincinnati's Bronson Arroyo lasted four innings, surrendering two runs (one earned) and four hits. Arroyo has not walked a batter in his last nine innings. San Francisco's Barry Zito worked 3 1/3 innings, yielding five hits and just one run. He escaped a bases-loaded, one-out jam in the third inning by coaxing Ryan Ludwick's one-hop smash to third baseman Joaquin Arias, who started a double play.
Arroyo and Zito welcome the extra preparation time afforded by the World Baseball Classic, which lengthened Spring Training.
"There's nothing worse than losing a game at the start of the season, because you've thrown 80 pitches only once," Arroyo said.
"I'm not where I want to be yet, but that's why we have [the time] we have," Zito said.
The Reds opened the scoring in the first inning as Zack Cozart doubled with one out and came home on Neftali Soto's single. San Francisco responded by scoring twice in the second inning after two were out with nobody on base. Peguero singled, Monell doubled him home and Wilson Valdez hit a high chopper to Arroyo that was scored as an infield single. Monell came home when Arroyo overthrew first base.
The Giants added a run in the fifth with more help from Monell, who homered in Tuesday night's loss to San Diego. He led off with a single against Sean Marshall, advanced to second base on a groundout and scored on Guillermo Quiroz's single.
Cincinnati pulled even in the sixth inning against Giants left-hander Javier Lopez on Derrick Robinson's RBI single and Chris Heisey's solo homer.
Up next for Reds: Right-hander Clay Hensley, who spent last season with the Giants, is among Cincinnati's relievers scheduled to work Thursday against Seattle at 4:05 p.m. ET. Hensley is competing for a spot in the Reds' bullpen. Homer Bailey, who has pitched just five innings in his three spring outings, will attempt to put together a longer stint as he makes his fourth Cactus League start.