[2] The Kansas City Royals selected O'Hearn in the eighth round of the 2014 Major League Baseball draft.

[8] He spent 2016 with both Wilmington and the Northwest Arkansas Naturals, slashing a combined .275/.351/.478 with 22 home runs and 78 RBIs in 134 total games between both teams, and 2017 with the Omaha Storm Chasers and Northwest Arkansas, batting a combined .253 with 22 home runs and 64 RBIs in 133 total games.

He was designated for assignment on December 28, 2022, creating a roster spot filled when the Royals signed Jordan Lyles.

[18][19] O'Hearn hit .375 with 3 home runs in 40 at-bats in spring training, but was ultimately reassigned to Triple-A Norfolk to begin the season.

The first was in a 10-inning 6–5 victory over the Toronto Blue Jays on May 20, highlighted by his three-run homer off Jordan Romano that tied the contest at 5–5 in the eighth.

[24] O'Hearn was cheaply acquired from the Royals, and as such was not expected to play regularly, other than as a left-handed pinch-hitter to face right handed pitchers.

His strong showing in these opportunities, with a .615 slugging percentage in May, as well as Ryan Mountcastle missing a month due to symptoms of vertigo, led to more playing time as the season continued.