OST files are stored on the local pc, not the exchange server. Therefore you cannot set a retention policy in Exchange for them. Once a user connects back to the exchange server of course that OST is going to synch back up and reflect what the mailbox actually contains on the exchange server itself, which would include whether items were deleted by a retention policy.
Are you trying to prevent users from using cached mode? That you can do with a group policy in AD, but that is not the same as a retention policy as I am sure you already know.