MLB umpire Brian O'Nora was arrested in a sex sting operation in Ohio this weekend ... after officials say he tried to solicit a prostitute online.
57-year-old O'Nora -- who's been an MLB umpire since 2000 -- was 1 of 14 alleged "Johns" who went online seeking to illegally purchase a sex act, according to police.
But O'Nora didn't get sex ... instead, he got nailed by the Liberty Township Police Department, which was working with a task force that specializes in busting human trafficking operations.
Details of exactly went down during the sting operation are unclear -- we're working on it -- but O'Nora and the others were all arrested and charged with misdemeanor soliciting and misdemeanor possessing criminal tools.