Last time I visited the UK, we had a 3 day London Travel Pass (bus & tube) and we went to visit brother-in-law in Oxford. While trying to purchase train tickets, a very helpful lady at the Underground ticket sales advised us to combine tickets to optimise the price and peak-off peaks. This time, we have an accommodation in Oxford for two nights and we (me and my wife) want to spend a day in London as tourists, hop on/off, visit sights etc. I am trying to combine the tickets to again maximise and optimise. My general view is: depart from the Oxford train station and take the 1st train after peak, spend the day in London and return to Oxford after peak, say 21h00. I'm looking for the best options and what type of London pass (bus, oyster, day pass). Any ideas will be helpful!
With the National rail enquiries you get a list of results for the train tickets. You will see the prices drop when you get to the 'off peak' time.
You can either search for a single/one way ticket or for a return journey as well, do that last.
In the results page for London trains you see a Travel card fares button.
Use that and the results are for a journey to London, return ticket and a travel card for all public transport in London.
Select an outward journey and the return journeys offered will fit that ticket or it will be indicated that you can use a train only if you buy a more expensive ticket.
The Traveline site is less useful for finding the cheapest tickets but might be better to plan for a door to door journey.
I think you can buy these tickets/travelcards as cheap on the day, by walking up to the ticket window. (I hope people with more experience can confirm it.)