Former Rotherham striker Reid scored five goals in his first 10 appearances this season before a serious thigh injury interrupted his campaign.
Marsh-Brown joined on a short-term deal in January, but the ex-Fulham youngster's initial promise on the wing was checked when he received a five-match ban for a red card and involvement in a subsequent brawl in the 2-1 home defeat to Scunthorpe in March.
Josh Parker, Djeny Bembo-Leta, Ryan Brooke, Matt Carr and Phil McGrath also depart, following Paul Black and Zander Diamond out of Boundary Park after it was announced last week the defensive duo would not earn new deals.
Fresh terms have been offered to nine out-of-contract players by manager Paul Dickov, with Kieran Lee, Robbie Simpson and James Wesolowski among the key figures he wants to tie down. A contract is on the table for Chris Taylor, but the long-serving midfielder is expected to leave the League One outfit having expressed a desire to test himself at Championship level.
Yousseff M'Changama, Kirk Millar, Filipe Morais and goalkeeper Dean Bouzanis are the others to have been given the option of extending their Oldham stays.
