From the official website:
"Supporters who wish to claim a refund on their match or coach ticket can do so in the following ways,.
If you have ordered online but have not collected your ticket, email supporter.services@oldhamathletic.co.uk by 4pm on Tuesday 23 April and your refund will be processed in 10 working days.
If you have already collected your ticket, please return your complete match ticket to the Latics Ticket Office by 4pm on Tuesday 23 April. The full ticket must be returned as proof of purchase.
Any refund for a coach seat must be claimed by using the email address above or in person by 4pm on Wednesday 17 April."