London to Oxford best transportation option

I took the Chiltern Railway from Marylebone Station in November and it worked well. I bought my tickets in advance because I needed to arrive and depart Oxford at particular times. Because of luggage, I grabbed a taxi from Oxford station, but I caught a local bus back to the station at the end of my Oxford stay. I’m sure someone who’s taken the Oxford Tube Bus will chime in here. I have no experience with the bus. Either the Marylebone or Paddington connections are easy enough to navigate. Just make sure your ticket is to Oxford Centre not the park and ride/Oxford Parkway (or whatever the second to last stop is called).

National Rail indicates that it takes about 1 1/2 hours by train from Oxford Circus to Oxford.
Bakerloo line to either Marylebone or Paddington stations and seems to be trains every 10 minutes between these two stations from early morning.
http://www.nationalrail.co.uk/

Trains between Paddington and Oxford are very frequent and take an hour to an hour and a half, from early morning to late at night. Schedules are on the National Rail website. Get a "day return" ticket (round trip the same day) for a small saving over two "single" (one-way) tickets. You can get it from a machine at Paddington or, if you want human help, a staffed window.

I'm sure there is bus service but I'm sure it will be slower and less comfortable, and subject to delays on the very crowded motorways near London.

The Heathrow Express runs several times an hour between the airport and Paddington, takes about 20 minutes. It's very expensive if you buy your ticket on the spot, less so in advance. Last year I bought a very inexpensive ticket about 90 days ahead, online, printing it out at home. It was nonrefundable and for a specific date, but for any train on that day -- a low-risk proposition if you know when your flight is.

I'm seeing some price variation on tickets from London to Oxford, with the Marylebone departures trending cheaper than the Paddington trains, but that may be different from day to day, for all I know. If you can snag an "Advance" ticket rather than being stuck with a full-fare "Anytime" ticket, you will save money, but you must be certain of your travel date and time. It varies by origin/destination, but sometimes train tickets bought on the day of travel are really painfully expensive. I don't know whether that happens on trips between London and Oxford.

A bus trip will normally take longer than a train, but often the bus is less expensive.

You can explore schedules and fares on the NationalRail website.

I have taken the Oxford Tube bus and it was fine in terms of comfort and price (very affordable, especially if you prebook) but the journey did take quite a long time to and from Oxford from Victoria coach station. I would recommend taking the train even though it is more expensive. Enjoy your trip; Oxford is magnificent. I highly recommend prebooking a tour of Bodleian Library (including the interior of Radcliffe camera). The tours are only available on specific days so be sure to plan it out prior to departure.

Thank you for all your input!
I'll definitely look at the National Railway as that seems like the best option!

The Heathrow Express runs several times an hour between the airport
and Paddington, takes about 20 minutes. It's very expensive if you buy
your ticket on the spot, less so in advance.

For last minute travel that takes 30 minutes between Heathrow & Paddington, you can take the Heathrow Connect for a much cheaper fare than HEX.

Advance tickets (with a capital "A" ie ones where you are tied to a particular train) are available starting from £5 for both the services from Paddington (GWR) or Marylebone (Chiltern); the absolute cheapest will be whichever is promoting them this week. The earlier you buy them the cheaper they will be although they remain towards the lower cost end even up to the day before if you are a bit flexible on the time. Since the fastest time on each route is comparable I suggest going one way and coming back the other for variety, as to Oxford Circus it makes little difference. For an early departure I suspect Chiltern might be easier to find a cheaper Advance.

I would normally only suggest the coach if you were staying in the immediate area you were staying in, which you aren't (Victoria, Marble Arch, Shepherd's Bush, Hillingdon etc). However, if your first point of call is The Kilns the first stop in Oxford for both coach services is Thornhill Park and Ride, which is in easy walking distance of The Kilns. So you have to weigh the slower time to Oxford against being able to arrive much closer to where you want to be without having to travel from the city centre.

I have taken both the train and bus between London and Oxford several times, and once I accidentally got on the wrong bus that stopped so many times I lost count. That seemed to take 3 hours. That being said, the train and express bus take about the same amount of time and go to the same destination in Oxford. The bus station is very close to the main train station. The bus however, was usually much less expensive at least when I was going back and forth. I did find it a bit difficult to locate the correct place to board my bus in London - the area is quite busy with many bus stops along the road.
As others have mentioned if your Oxford destination is closer to the bus stop that might be worth considering, although there are local bus services in Oxford as well. Maybe consider the train one way and bus on the return? (or the opposite)

If you can, it is best to buy the tickets in advance online and not the day of. For many train tickets you can print them at machines at the stations using a code and the credit card you used to pay for them. But for the bus you can also pay the driver, at least the one I took.

Oxford is one of my favorite places. So much history. The Pitt Rivers museum is my favorite.

The bus to Kiln Lane (9 - Risinghurst) from the centre leaves from the Westgate Shopping centre twice an hour. An somewhat unusual feature is that the operator of the bus alternates between Stagecoach and the Oxford Bus Company. Doesn't matter if going only one way but if going there and back, buy a SmartZone 24 ticket, valid on all buses operated by those companies (plus Thames Travel) within the city for 24 hours.

we booked advanced tickets on a rail and was very reasonable

Another vote for the train - we did not book in advance, but it was reasonable for us. We find the train a very relaxing way to travel.
PS - I am a senior (they call it pensioner), so qualified for a discount ticket.

I prefer taking the rail over the bus, but if I do it looks like I'll have to bus from the train station to "The Kilns", which is about 40 min.
Bus might be more practical because there are stops nearer to my destination. I just hope I can find the actual bus stop and not get lost!!!

What I could also do is try to get a late afternoon tour of "The Kilns". That way, I can take an early train to Oxford, walk around the city for a bit then take the local bus up to The Kilns.

Is the local buses difficult to figure out?