This is very easy to do yourself. Trains as above, Oxford station is right in the city centre, and if you Google "oxford self guided walk" you will find lots of options. Enjoy!
Organised tours usually spend two hours in Oxford en route to Stonehenge or Stratford-upon-Avon - that's barely enough time to walk from the coach park, use the toilet, buy a coffee and take a selfie outside Christ Church.
Oxford deserves a full day (at least!) and the train takes around an hour from London Paddington or Marylebone. Small, easily walkable city, lots to see. Visit the tourist information centre when you get here and pick up a map / walking guide, but please resist the HoHo bus - it misses some of the prettiest bits of the city centre because it's pedestrianised.
I concur with beckysimms - and if you search this forum there's details of lots of self guided tours.
I can only agree with beckysimms and pcw12kcox - they are the experts for Oxford. Plan where and what you want to see, its not a big City and extremely easy to walk around, in fact its the easiest way to get around. Contact the tourist info centre in Broad Street, for further help and planning.
As an tourist, who has visited Oxford many times. The best way to do it is to take the train to oxford and then as everyone has said go to visitor information. I like walking. So many interesting things to do and on your own time.
Re reply 6 there's also this bus service: https://x90.oxfordbus.co.uk/ There is little difference between the rival services. However be aware that although both bus services can be cheaper and more frequent they will take longer than the train (normally at least 35/40 mins extra).
Be also aware there are trains from Paddington Sation in London (as mentioned in the first reply above) AND Marylebone Station (which take about 5 mins longer, but the service tends to be a bit more reliable as it uses a different line to the Paddington train).Edited: 11 June 2018, 04:33
Thank you so much for all your advice. It looks like train and walking tour for us. Also means we can 'do our own thing'. Now to get organised.