buying rail tickets to London

When is your trip? If it is more than 90 days away, tickets won't be available for sale.

If it is less than 90 days away, have you tried going to Great Western Railways site directly?

For Oxford to London, you have a choice between Great Western Railway and Chiltern Railways. The Chiltern route takes maybe 15-20 minutes longer and goes to Marylebone which has fewer Tube connections (although Baker Street is nearby), but it could be cheaper.

There are actually 3 good choices from Oxford to London. In addition to the GWR into Paddington and Chiltern into Marylebone, you can also consider The Oxford Tube which is a coach from 4 Green Coach Station in Oxford to Victoria Coach Station in London. Frequency as much as 10 minutes between, and with 2 express runs. There are 4 pick up stops in central Oxford, so very convenient, and Shepherd's Bush, Notting Hill Gate and Marble Arch in London, so plenty of tube and bus choices, on its way to Victoria. For reference: https://www.oxfordtube.com/

It really depends on where in Oxford you will be, and where you will be going in London, time of day, etc. Plenty of choice.

I am surprised that you required to reserve a seat.

National Rail doesn't sell the tickets yet, they send you through to a train company don't they? At some point we will have the thrill of nationalised rails again, or what the government want us to think is nationalised rails, but your post is very surprising to me.

Please share date and time, how many in the party, and number of seniors and under 15s. I have decades of experience selling train tickets (in the UK I worked for 4 train companies), this is easily sorted out.

Were you buying Advance Tickets (the specific name)?

I just gave it try with a “dummy booking” and had no problem.

I chose the GWR service with no changes. When it said I am required to have a reservation, I chose to “reserve.” With the button. The next window offered seating preferences but I chose none. Next page showed me my assigned seat (it was something like “ carriage B, row 16, window seat”.

Then I was told to select “pick up at station” for the ticket delivery* and chose Oxford Station for the delivery.

The window after that asked me to log into my GWR account, which I do not have, so I stopped there.

This was an Advance ticket (price £10.40) for August 31, 5 days from now.

*This was required because the travel date is so close to purchase date.

Edit: I am guessing that the reservation requirement is to enforce social distances, by assigning widely-spaced seats.

Thanks to all for your helpful replies! Is October 19, less than 90 days away. I don't understand the hangup on my end but now I have some very good options. Thanks again! I should look further as to where we will be in Oxford as we are returning our car there and I have no idea about the geography of that city and proximity to train stations. I may be back with more inquiries ,!

most but not all rental offices are at the main Oxford station. Leave a couple of extra minutes, the entrance to the car park area at Oxford station is a bit tight if I remember correctly.