Peter Robert O'Brien (born July 15, 1990) is an American professional baseball left fielder for the Pericos de Puebla of the Mexican League.

He has previously played in Major League Baseball (MLB) for the Arizona Diamondbacks and Miami Marlins.

Unselected in the 2008 MLB draft, O'Brien chose to enroll at Bethune–Cookman University, where he played college baseball for the Bethune–Cookman Wildcats team in the Mid-Eastern Athletic Conference (MEAC).

[17] The Diamondbacks assigned O'Brien to the Reno Aces of the Triple–A Pacific Coast League (PCL) in 2015, and converted him into an outfielder.

[19] O'Brien made his major league debut on September 11, 2015, at Chase Field against the Los Angeles Dodgers.

O'Brien was called up to the big league club on June 10 after hitting .330 with 17 home runs and 52 RBI in 212 at-bats at Triple-A.

[24] The Diamondbacks traded O'Brien to the Kansas City Royals for minor leaguer Sam Lewis.

[25] Following the Royals Spring Training camp, O'Brien was sent back to the minors and briefly played for the Omaha Storm Chasers of the PCL, before he was designated for assignment on May 10, 2017.

[26] On May 16, 2017, the Cincinnati Reds claimed O'Brien off of waivers and assigned him to the Louisville Bats of the Triple-A International League.

[29] After playing in 16 games for the Triple-A Round Rock Express and Double-A Frisco RoughRiders, O'Brien was again designated for assignment on June 17, 2017, when Ernesto Frieri was selected to the roster.

[39] O'Brien returned to the majors on May 4[40] but suffered a rib injury just a few days later, sending him to the Disabled List.

O'Brien signed a minor league contract with the Atlanta Braves as a non-roster invitee before the 2020 season.

[51] O'Brien hit a go-ahead home run in Game 7 of the Serie del Rey, which culminated in a 3–0 win and the Toros' second league championship.