Yes but it will take a long time.
It all started way back, a long, long time back etc
Historically short journeys of up to about 20 minutes had a day return ticket - longer journeys had an open tyoe ticket. So for example Chippenham to Bath Spa only has day return but Swindon to Reading has the Off Peak (Open) Return.
But wait a minute you say what about Oxford to London that is about an hour but has Peak and Off Peak day return tickets. Well back in the 1980's the railways around London were all run under one brand Network Southeast by a nice bloke by the name of Chris Green. He wanted to encourage off peak travel so off peak day returns were introduced across the area. However Swindon was not in the Network Southeast area so no day return was introduced.
It was about this time that Peak tickets from Oxford became Day tickets not Open tickets because he abolished all barrier checks. Whilst this was nice for the passenger it was not very good for revenue control as on the busy trains in the peak it was not physically possible to check tickets on the train and open tickets could thus be reused the next day ! thus the only more than one day return from Oxford to Paddington is off peak when in theory it should be possible to check the ticket on the train. Of course barriers have now been reintroduced but that is another story !
So really it is a historically reason that you cant get an Off Peak day return from Swindon to Reading. Hope that helps but probably raises more questions that it answers.