Off peak train tickets

1) Yes you can get a flexible return- Anytime Day Return £73.80 or Off Peak Day Return £25.50 valid on any train out after 0913 Monday to Friday, any train at all at weekends, or the advance out and a Super Off Peak Single back at £22.70 (which has the odd evening train it is not valid on).
2) You can buy at the station or on your device until a minute or two before the train is due to depart
3) afternoon restriction only applies to the 1515, 1616, 1716 and 1817 and 1916 trains
4) No, same price through any sales channel.

Many people here swear by LNER as the easiest to use site for Americans- even though you are not travelling with LNER.

All fares quoted are before any relevant railcard discount of 1/3rd (Senior, Two Together, 'Disabled', Family and Friends, under 26)

https://www.lner.co.uk/ A big part of LNER's ease of use was because non-UK card payments would go through without issue at a time when a number of other train operating companys' websites didn't like foreign cards. I don't think that's so much an issue as it use to be.

GWR would be a train operating company for that route if it makes a difference. https://www.gwr.com/

You could just go into Bath station and ask for an off peak day return to Oxford = £25.50. Note that you will have to change trains at Didcot Parkway so be ready to leave your seat when they announce approaching Didcot Parkway.

On Saturdays only, a direct train starts in Bristol and departs Bath at 10.31 for Oxford. I think this is a bit late for your day trip but you might like to use a direct service back which leaves Oxford at 17.12 arriving Bath at 18.09.

I tried googling but I couldn't find all of these train routes: "1515, 1616, 1716 and 1817 and 1916 trains"

Is there a map to show what those routers are?

those are departure times - 15 minutes after 3pm etc.

24 hour clock, widely used here, 1500 is 3pm, 1600 is 4pm, etc.

If you type in for a journey on the following site - https://www.traintickets.com/?/- it will tell you the times and prices but it will also produce a diagrammatic map of the route. The train from Bath to Didcot will be going to London as final destination. All will be clear at the stations with signage and you can always ask staff or just other travellers.

If you have to change at Didcot Parkway, trains to Oxford normally go from P3 or 4 (with some continuing to Banbury). If you have to change at Didcot on the way back to Bath, go to P1.

Here is a diagrammatic map of the entire system. Scroll down for the south.

Wait a minute. So, you're telling me that with this off-peak weekday train ticket , you:

can't take the train from Oxford to Bath that leaves at 16:16 but you could take the one that leaves at 16:04

can't take the train from Oxford to Bath that leaves at 15:15 but you could take the one that leaves at 15:07

Not going to lie, this system sounds a bit strange and confusing for an outsider like me.

Is this how all the off-peak train tickets work, or is this just specific to this Bath-Oxford line?

can't take the train from Oxford to Bath that leaves at 16:16 but you could take the one that leaves at 16:04
can't take the train from Oxford to Bath that leaves at 15:15 but you could take the one that leaves at 15:07

It'll be because the :16/:15 trains are Cross Country where the change to Bath takes place at Reading. The :04/:07 trains are GWR and the change to Bath takes place at Didcot Parkway. The particular Off-Peak tickets discussed are generally GWR service, "Not valid for travel via (changing trains or passing through) Reading."

Different services and different routes may use the same tracks and stations.

This is a bit of an odd case, as Reading is further east than Didcot. If the Cross Country service stopped at Didcot on the way from Oxford to Reading then the off peak ticket would be available on their trains between Oxford and Didcot- the approx. quarter past every hour train

So the problem is the route, not the operator. So not quite the same as VRE vs Amtrak or Metro North vs Amtrak (in New York State).

As you were talking about saving money I looked at doing train from Bath to Didcot, then the citybus from Didcot to Oxford, but you would only save £2.50 by doing that as an Off Peak Return from Bath to Didcot is £19. And the bus fare each way is £2.

If it was a one way trip, not a day return you could save money by using the train to Swindon (£14.50 Single, £14,60 return), then the bus. But the bus is not very frequent and takes too long for a day trip.

So £25.50 is the lowest available fare, without railcard reductions.

If you change at Reading, you will travel through Didcot firstly on the way there, and then a second time 20 odd minutes later on the way to Oxford - it adds on a large backtracking loop to the journey even if the journey times are not quite as different as changing at Didcot.

Now some Advances have been routed that way in the past but they don't seem to be sold since the timetable was recast last year, possibly because there is more than enough travelling on the remaining Cross Country services.

Cross Country Trains are long express distance trains that don’t go to London. So, the ones that go through Oxford will have come from somewhere north of Birmingham and will usually be travelling down to the south coast - say Southampton. They go non stop between Oxford and Reading - skirting Didcot. So, it is all quite logical that the lowest fare is for Bath to Didcot Parkway and then change for the short hop on a local GWR train up to Oxford.

A good source for train travel, including the hunt for bargains, is usually www.seat61.com, apparently named for his favourite seat on the tunnel train.

A new way to do this cheaply is train to Chippenham or Swindon then the £2 buses.
Bus 55 from Chippenham station to Swindon every 30 minutes by day - first out at 0545, last back at 2315 arriving at 0024.
Then the new S6, Swindon to Oxford, takes 80 minutes. Every 20 to 30 minutes. Last connecting one back at 2245, arrives Swindon 2357 for a last 0039 train to Bath which also calls at Chippenham.
Or the 2145 would connect to the last Chippenham bus.
So this can be done very affordably and still give you lots of time in Oxford, with no debates over which train is valid on the way back.