(I don't live in the UK, and I have no idea of what an Oxfordshire is.)
How you get to England is up to you. Try to get a road map of Britain before you arrive, or pick one up at the airport, station or ferry port. Even if you are not hiring a car, at least you'll know roughly where you are.
You need to get to a town called Banbury. There is a railway station there, and some trains from London or Birmingham will stop at Banbury. You can get a train to London or Birmingham from anywhere. Check the map, decide which is nearest to you, and go to the nearest railway station to buy a ticket to Banbury. The staff at the station will be happy to tell you which train to catch, and where to change to another train.
British trains are probably not as good as the ones you are used to. We're working on it. Very slowly.
Once you get to Banbury, ask the staff at the station where you can catch the bus to Cropredy. During the festival, buses to Cropredy are frequent.
If you have hired a car, you need to find the M40 - it will be a blue road on your map. The M40 goes to Banbury, and when you get to the turn-off (or off-ramp, for US pals) marked "Banbury" - take it. At the roundabout, circular gyratory system, call it what you will, that you will come to, you will find signs to "Music Festival". Follow them, and you will shortly find yourself driving down country lanes that become alarmingly narrow. Have no fear, within twenty minutes or less you will be in a queue. It's the queue for parking and camping at Cropredy, and it usually starts in or before the next village, Williamscote. Turn the engine off if the queue shows no sign of moving, wind a window down and enjoy the silence. Relax... You're nearly there. If you prefer a little music, Leige And Lief works well at this point.