Trains out of Heathrow

I think it's feasible. It should not take you more then 2 hours to go through customs and airport processing

Is that time you are being quoted for the train at Paddington, or for the Heathrow Express train to Paddington?

I'd not be surprised if that can be too tight.

You should be able to take the underground between Terminal 5 & Terminal 4, here is the information on the process and costs: https://www.heathrow.com/transport-and-directions/underground.

Nigel- the £26 with two changes goes through Hayes and Harlington and then Slough. The more expensive goes through Paddington and that was quoted as leaving from Heathrow.

You cannot take the Underground between Terminals 4 and 5. For the cheaper tickets, you would have to take the Heathrow Express to Terminal 4 then then TFL rail to Paddington then get a train from Paddington to Oxford.

Here's an alternative. National Express offers direct bus service from Terminal 5 to Oxford. It takes 90 minutes and you don't have to backtrack into London. The fares are as low as 24 GBP. There is a bus every half hour. No changes and multiple stops around Oxford.

It looks like the way you are quoting is a bus from Heathrow to Reading then a train to Oxford. If you're getting on a bus anyway, why not just take the direct one?

The £52 fare takes you on the expensive Heathrow Express from Terminal 5 to Paddington. Then you need to change trains for Oxford.

For the £26 fare you need the TfL rail service then a couple of changes.

Alternatives are, take the Rail Air coach from Heathrow to Reading then a train to Oxford (use ‘Heathrow Bus’ to Oxford on the national rail journey planner).

https://www.firstgroup.com/berkshire-thames-valley/rail-air/

Or take a frequent coach from Heathrow direct to Oxford

https://airline.oxfordbus.co.uk/

https://www.nationalexpress.com/en

The least hassle are the coaches.

The only issue I see with your plan is that you really can not control is when your plane will arrive plus the amount of time it takes to clear passport control at Heathrow. Unless you are buying a fully refundable ticket, I would wait until you land to purchase your tickets.

Yea, I’ve thought about possibly purchasing there day of. Won’t have checked bags, for the first time ever I’ll be travelling light! So may not take as long as usual to get through everything.
Considering buses now....will see how purchasing bus day of works, if it works.
thank you all.

We had a good experience with The Airline, one of those recommended above by ramblin' on. We bought the ticket online for a specific time, a couple of hours after landing as I recall, but we were able to use it on an earlier bus with no problem. It was a pleasant and pretty fast ride to Oxford outskirts, a few suburban stops, then a block from our hotel in the middle of the city. Trains are usually the best way between cities, but Heathrow doesn't have direct train service other than into London.

Go to www.nationalrail.co.uk

From Heathrow > OXF
Now select More Options and then Route. Change Travel Via to AVOID > PAD. By doing this, you avoid the system telling you to take the expensive Heathrow Express into Paddington and then out west again. What you should now find is a pay on the day fare of £31.20 peak or £26 for departures after 9.52am on weekdays and all weekend. The you click DETAILS it will tell you what trains to catch - usually you go from Heathrow to Hayes & Harlington and then west to Reading to change for places to the west.

In 2016, I avoided all the switching around by taking the direct coach from Heathrow to Bath. I waited to get my ticket until after I arrived. I don't remember the walk up price being different from buying in advance. The ride was fun and I enjoyed going through all the little stops along the way.

I suspect that doing something similar for Oxford would produce similar results. I'd take the bus (coach).

Going from Heathrow to Oxford, check out the Oxford Bus Company. You can download their app and purchase a ticket, but I wouldn't worry too much about purchasing ahead of time. If you don't want to mess with the app you can buy a ticket from a kiosk (or even the driver, if you have the correct cash). A return ticket instead of a single would only cost you a little more, you can use it when you need to, and you could be back at Heathrow very cheaply either for a flight or to head on into London.