Thanks Graham and Chris, big help. That bit about the Chiltern lines is super-useful, I would never have thought of that.
Excellent. The ONLY downside I can see of having an Oxford season rather than a Didcot one is if you wanted to get the train at Paddington one evening and stay on to SWINDON or beyond - with a season to Didcot you could take any train including one that did not stop and Didcot, whereas with an Oxford season you might be limited to trains calling at Didcot.
14.2. If you are using a Season Ticket, daily Zonal Ticket, or another area based Ticket such as a concessionary pass, ranger or rover in conjunction with another Ticket and the last station at which one Ticket is valid and the first station that the other Ticket is valid are the same, then the train does not need to call at that station for your combination to be valid.
Even there, you could argue with an Oxford season that the last station it is valid at [on the journey to Swindon] is Didcot ...
Also to note (in addition to the original answer) that the ticket is to "London Terminals" and that Oxford or Didcot to London services will also let you travel in to Waterloo via Wokingham, Staines, Clapham Junction ....