Brownless, son of club great Billy, joined the Cats via the 74th selection of the 2018 National Draft.
The 21-year-old was unable to break into Chris Scott's side during his tenure at Kardinia Park.
Speaking on the club's latest list call, Geelong’s general manager of football Simon Lloyd thanked Brownless for his service to the club.
“Oscar is an outstanding person, and we thank him for his contribution to the club,” Lloyd said.
“Oscar’s warmth, positivity and professional attitude saw him form many strong relationships over his three years at Geelong.
“We look forward to seeing him succeed in his future endeavours.”
The delisting of Brownless leaves midfielder Charlie Constable as the only Cat yet to recommit to Geelong.