Best Option from London to Oxford for a Sunday Day Trip

Trains to Oxford leave from London Paddington (not St Pancras) or Marylebone.

From St Paul's to Paddington I would take the Circle Line from either Mansion House or Blackfriars to Paddington (both a 5 to 7 minute walk from St Paul's), the same to Victoria for the Oxford Tube.

Being a Sunday you would need to check for any engineering work on the rail line that might disrupt the journey.

To Marylebone Central Line from St Paul's to Oxford Circus then the Bakerloo line.

Especially if there is rail disruption the Oxford Tube is a perfectly good alternative, and cheaper as well.

Train from Paddington is hourly only Sunday morning, journey time from there 1 hour. Likely to be the most expensive.

Train from Marylebone is twice an hour but takes 20-30 minutes longer.

Oxford Tube is three times an hour but takes up to 1 hour 50 minutes.

There are arguments for & against all of these.

Depending on exactly where you are staying walking to the Barbican entrance to the Elizabeth line could be the best way to Paddington.

It’s worth noting that Oxford Station is about three quarters of a mile from the centre of Oxford whereas the Oxford tube has a stop right in the centre. This might be worth factoring into any issues of journey times.

High Street is probably the best stop to alight rather than wait until the end (Gloucester Green bus station).

What Victoria Station is the departure/arrival point for the Oxford Tube Bus, there are more than one. I would be taking the Circle Line from the Mansion Underground Station near where we are staying and then take an Oxford Tube Bus each way.
It takes longer than the train, but Victoria Station is closer than traveling on to Paddington Station and I like the idea of leaving the bus closer to the city center. Also, the Oxford Train Station has many bad reviews claiming it is dirty and unpleasant.
We likely would buy our bus tickets online a few days before. Are the seats reserved? Can anyone comment on the bus ride itself?

The Oxford Tube leaves from kerbside on the road between Victoria Rail Station and Victoria Coach Station.

There are no reserved seats.

If you are a Senior it is £20 return, not senior £22 purchased on the app, £1 more purchased on the bus.

Is it cancellable? Time specific if you are late for the time you selected?
Also, what is the activation 15 minute limit mean?

Personally, for a double deck bus which runs every 20 minutes I would not buy a ticket in advance, as it is time specific. That just seems to me to be setting myself up for problems. These are the longest buses in the UK by length so have high capacity.

I don't do bus tickets on an app but guess it means you don't activate the ticket more than 15 minutes before use.

This is from the FAQ's about cancellations.

Unfortunately, you can't change your ticket but you can cancel it if it hasn’t been activated. After cancelling it, you can request a refund and buy a new one. If you haven't activated your ticket, we'll send you a full refund. If you've already used part of it, we'll send you the remaining value after taking a £15 early cancellation charge. To cancel your coach ticket online:

1. Log in to your stagecoachbus.com account
2. Go to ‘My tickets’
3. Select ‘Show details’
4. Choose the ticket you want to cancel and select ‘Cancel online’

To also cancel your ticket you can email [email protected] or fill out our Refund request form

"Oxford Train Station has many bad reviews claiming it is dirty and unpleasant" - where did you read that rubbish?

It's about a sevin minute walk to the edge of the inner town.

Oxford Railway station can get crowded and could benefit from being redeveloped (which is in the early stages of happening), but what interior space there is has been opened out somewhat already in recent years. Can't recall personally finding it specifically dirty in all the years I've used it.. The bus station is if anything a bit more seedy but still nothing to be that concerned about for me.

The Tube advertises itself as a turn up and go service although buying via the app saves a pound. Make sure though that you don't buy a child ticket online as curiously they are listed first.

Can you buy a return ticket on a walk up basis when you first board to make an onward journey. If so, must you be time specific for your return time later the same day?
There is a good savings buying a return for a same day trip on the app. I don’t mind a pound extra buying walk up, but if you must pay that each way for two singles and not have the round trip savings it seem a bit much.

From the Oxford Tube FAQ:

Single and return tickets can be purchased direct from the driver on the day of travel.

for a day trip I would much rather 2 hours on the train - an hour each way - than 4 hours on a bus - nearly 2 hours each way, longer in traffic.

Are you sure it wasn't the bus station that was dirty and unpleasant? I've never seen that at either Paddington or Oxford station. And there are toilets on the train.

Actually, the train looks better and better. For 2 adults with Two Together Rail card the price is always the same and we can buy it that morning online and collect at station. Also, seems to allow us to possibly take an earlier or later return train should we choose.
The reference to the cleanliness of the train station came from several online reviews.

Question? I can buy two adult, return London Paddington-Oxford tickets with a Rail Card for about $59.60 on any Sunday over the next few months that I have looked at without any savings buying the tickets several days in advance. As a “Off Peak Day Return Ticket”, may I return on any afternoon train that day or only the specific time train I book?

Yes, any afternoon train to Paddington (not to Marylebone, as that is a higher priced route)

So that would be any afternoon train back to Paddington from Oxford?
We would be taking a morning train from Paddington to Oxford.

Yes, total flexibility of train in both directions.

If you wanted to, on that ticket, you could break your journey at any station in either or both directions.

Can I ask another train question?
We plan a Monday trip from London to Hampton Court Palace. We will take the Tube from Bank Underground Station (near our rental) to London Waterloo Station and then the 9:36 - 10:12 a.m. SW Railway Train to Hampton Court and a return train back mid. to late afternoon that day finding the best price for 2 adults, return with a Rail Card to be $28.18 which we must collect at London Waterloo before departing. This is a “Off Peak Day Travel card” and it seems to allow us to use it on the Tube when we return from Waterloo Station to Bank, is that correct? May we take any afternoon train back from Hampton Court to London Waterloo or is it time specific? This ticket price is irrespective of how far in advance it is purchased.

Our other tickets’ option for the same trip is to purchase “Anytime Day Return Tickets” for a few dollars more, $31.56, which can be purchased in advance or the same day, no advantages, and it appears to allow us to return on any afternoon train to London Waterloo that day. It also allows for purchase as an e-ticket rather than collecting from the station. It does not allow us free use on the Tube as the above Rail Card does.
Is my analysis of our two choices above correct?

Yes the Off Peak Day Travelcard is your best option. That can be bought on the day if you want, rather than pre-purchased.

That is totally flexible to use on any train after 0930, and can be used on as many national rail trains, tube trains and buses as you wish that day until close of service, within Zones 1 to 6.

From Bank to Waterloo take the Waterloo and City line- a straight shot between the two stations with no intermediate stations.

I don't believe that the Drain - the Waterloo & City - have a Sunday service.

Thanks much for all the help, I have learned much!

Regarding Nigel's comment about the Waterloo and City not having a Sunday service, it doesn't matter when the OP plans to use it on a Monday, per their specific question.
Since the Pandemic it hasn't run on a Saturday either.