I tried looking for train tickets from Bath to Oxford one way in June and from Oxford to SAV one way but the site does not seem to work for one way. There is a box that says "Are you returning". I leave it blank, if I click on it the box for a return trip pops up. But it will not proceed when I do not click the box. I have tried over a variety of days, thinking there was a problem, but it is still doing that. Can I not look for one way on that site? I actually am doing the trips in July but too early to look for then. Some posters say to go to the rail site for the best information. I can do it one way on the National Rail site.
I'm not sure of the problem you're experiencing on the GWR wehsite, however, your ticket options on the GWR website should be single, return, and open return. A single ticket is your one way ticket option and choose that if its not selected by default. I did a few dummy booking on the GWR website for single tickets without an issue.
I tried the site again and was directed to another page--that one had the single, return, spot. Thank you for your help.