O'Hearn was optioned to Triple-A Omaha on Friday, Jeffrey Flanagan of MLB.com reports.
O'Hearn was 4-for-26 with 11 strikeouts since the start of June, dropping his season slash line down to .188/.286/.333. Lucas Duda and Cheslor Cuthbert should see time at first base for the Royals, at least until Hunter Dozier (side) returns from the injured list.