The most convenient airport for Oxford is London Heathrow - the Airline coach leaves from the Central Bus Station (Terminals 1, 2, 3) every 30 min (20 min earlier from Terminal 4) and takes just over 1 hour.
From London Gatwick the Airline coach leaves every hour (stopping at both North and South terminals) and takes just over 2 hrs.
If arriving at London Luton or Stansted airports, it takes 3-4 hrs to get to Oxford by National Express coach (typically via London or Heathrow) so this is not recommended.
There are two frequent coach services to/from London - the Oxford Express and the Oxford Tube - taking ~1.5-2 hrs. Trains to/from London Paddington typically take ~1 hr (if arriving by Eurostar you can take the "tube" from St Pancras to Paddington).
Do not consider driving as parking is both difficult and expensive in Oxford (neither Keble College/Rewley House/University Club nor the Physics Department can provide parking although the hotels might). If you wish to take a taxi from the airports (convenient for Luton or Stansted) it is cheaper to prebook one from a local company (e.g. CCB Cars, Executive Travel, OxiCars) who will meet you at Arrivals.
If coming by coach from London/Heathrow/Gatwick/Luton/Stansted, stay on till the final stop at "Gloucester Green" Bus Station - this is a ~10 min walk from the Workshop venue on Keble Road (or you can take a taxi for ~GBP 4).
If arriving at Oxford by train, you can walk from the station (~20 min) or take a taxi (~GBP 5) or Bus 14/14A (get off at "St Giles").
(If taking a taxi ask to get down at "Mathematical Institute, St Giles" and walk around the Keble Road corner to save time as this end of Keble Road is closed to cars)
See How to Get to Oxford University for more details and maps.