The defender joined the Hornets on dual-registration forms from Dorking Wanderers during the curtailed 2019-20 campaign, and made 18 appearances for the club.
Horsham made the move permanent last summer, and O'Sullivan made a further eight appearances, scoring once, before the 2020-21 season ended prematurely.
The full-back is the sixth player to depart The Camping World Community Stadium in the off-season.
Arron Hopkinson, Reece Hall, Kieran Monlouis, David Ray and Lewis Taylor all left the club last week.
Posting on Twitter, O'Sullivan said: "Thanks to everyone involved at Horsham FC, fans, staff and players, all top class people. It’s been a pleasure.
"Good luck for the season and thank you for everything."