O'Brien attended Shorewood High School in Shoreline, Washington, where he played baseball.

[4] After the season, he was selected by the Tampa Bay Rays in the eighth round of the 2017 Major League Baseball draft.

[9] In 2018, he began the season with the Bowling Green Hot Rods of the Single–A Midwest League (with whom he was named an All-Star) before being promoted to the Charlotte Stone Crabs of the High–A Florida State League, with whom he ended the year; over 25 games (13 starts) between both teams, he went 8–4 with a 2.75 ERA.

[19] On September 28, 2021, the Reds recalled O'Brien to the active roster to make his MLB debut versus the Chicago White Sox.

[20][21] He was the starting pitcher and threw 1+1⁄3 innings, giving up two solo home runs while walking three and striking out two.

[23] On April 16, 2022, the Reds traded O’Brien to the Seattle Mariners for cash considerations or a player to be named later.

[24] (On June 3, minor league infielder Luis Chevalier was sent to Cincinnati to complete the trade).