The complaint was lodged by the Association of Convenience Stores and Postmasternetwork, which represents subpostmasters, in November last year.
The ACS claimed c-store subpostmasters were often losing out on the best deals because the Post Office was forcing them to only provide services from its preferred suppliers.
The ACS also said the Post Office was claiming unfair margins on some services provided by subpostmasters, such as the National Lottery.
In its interim ruling, the OFT said the Post Office had taken steps to lessen the restrictive nature of its contracts. It added that it had to maintain some restrictions to ensure the continuation of the Post Office network.
But the ACS dismissed that view, saying the Post Office could also rely on government subsidies.
The ACS and Postmasternetwork now have until the end of the month to respond.