I often travel Oxford to London offpeak, so it'd be convenient if I could pre-purchase a stack of anytime offpeak tickets, rather than having to queue when I get to the train station (they only have a few machines). Is there a way to purchase these in advance without specifying a specific day/time? (If I knew the exact time I'd purchase an Advance ticket, but I usually don't.) I can't find anything on nationalrail.co.uk or gwr.com, but I haven't asked yet at the ticket counter in the the station.
It's possible to get a London to Oxford carnet ticket - this is a book of 10 anytime day return tickets for the price of 9 valid for three months. However, it's quite hard to find information about this and I don't know much more about it. The tickets show up on BRFares and they seem to be priced by GWR - this would indicate to me they're valid on both Chiltern and GWR but I don't know for sure. Chiltern advertise them as being available for purchase through Marylebone ticket office, and GWR mention their existence but they seem to be available only through their business portal.
I would ask Chiltern and/or GWR's customer services for more details. Maybe you can get what you want. Do report back with your own answer here once you've figured it out!
It is not possible to buy an off-peak ticket without specifying the day of travel. In particular there is no system of "validation" as there is in France for example.
As comments have discussed, you might find that a Mobile eTicket resolves your issue, though this is not universally true across the network.