GWR▸ gateline staff at Oxford (when the ticket office is closed) have refused to sell me the Oxford Evening Out, claiming that it has to be bought from a 'country' station and not from Oxford. Which sounds like hogwash and isn't reflected in any online documentation that I can find. (I'd cycled from Charlbury to Oxford and was returning by train.)
Indeed. As far as I can see you should even be able to buy it at the ticket office at Wolverhampton and split onto it at Banbury.
As a leisure ranger / rover product you should also be able to use it to split at Didcot if you've got a regular (single or return) ticket to Didcot from somewhere south or east thereof, even if you're passing Didcot on a train that doesn't stop there.
A Ranger ticket which is valid for 1 evening, offering unlimited travel in the area shown on the map.
and
This ticket can either be bought on the day or in advance from a station ticket office, online, by telephone (08457 000 125), or on the train (valid ticket/permit to travel is required unless boarded at a station without ticket-issuing facilities).