I suspect they may be able to detain you with the assistance of RPIs▸ or the BTP▸ until they can verify your Oyster▸ should you get an obstreperous TM‡
I suspect they would be acting beyond their legal authority if they did.
The legal requirement is to produce a valid ticket when requested. An Oyster card with a Season ticket on it, or one with
PAYG▸ credit for a journey within the zones that has been touched in, are both valid tickets. Your legal duty has been discharged if you present it for inspection. Oyster is merely the medium on which the ticket or fare is held. If on-train staff have no way of checking that medium then that's their problem. Without the ability to check there should be the presumption of innocence. BTP will be mightily hacked off if they attend at the request of on-train staff only to subsequently discover the Oyster card held is perfectly valid.