Getting to Oxford:
Airports:
There are a number of possible airports. Of those in London, Heathrow is just over an hour away, Gatwick and Luton are about two hours away. Stansted is about two and a half hours away.
Also good options are Birmingham (1hr), Bristol (1:30).
In order of preference: Birmingham or Heathrow; Gatwick; Bristol; Luton; Stansted.
From the airport:
From Heathrow, take the Airline bus. This can be found in the coach departures in the central bus station in Heathrow (near terminal 2). It also stops at Terminal 5. If you end up in Terminal 4, you will need to get a free bus connection to Terminal 5.
From Birmingham simply take the direct train from the station to Oxford. Get an off-peak return ticket from one of the machines. (For advanced users one can `split-ticket' via Coventry/Leamington Spa or Banbury to get a cheaper ticket. This must be planned in advance.)
From Gatwick, get either the Airline bus, or the train. For the latter, see here; one gets a slow train to Reading, then there are fast connections to Oxford. If buying tickets at one of the many machines at Gatwick airport, select Oxford as the destination, choose `Off-Peak Return Not Via London'. This will have a return valid for a month. If you arrive earlier than 9am, you many need to choose `Anytime Return Not Via London'.
From Bristol take the Airport Flyer Bus from the airport to Bristol Temple Meads train station. Get an off-peak return to Oxford. You may need to get a train to Bristol Parkway and change. See nationalrail.co.uk for details.
From Luton, get the National Express Coach if possible. It is bus 737 and leaves from the front of Luton airport. Timetable is here. Notice that there is often a big wait between buses. If you have just missed one, then get the train. Buy an off-peak return ticket to Oxford. You will arrive in London King's Cross/St Pancreas; your ticket will get you through the underground to Paddington; from Paddington there are many trains to Oxford.
From Stansted do not get the coach. Get a train ticket off-peak return to Oxford. You will arrive in London Liverpool Street; your ticket will get you through the underground network to Paddington (take central line to Oxford Street and change to Bakerloo Line); from Paddington there are many trains to Oxford.
From the train station to Somerville:
See map. Go straight out of the station, cross the road, turn right and follow the building round, turning left at the traffic lights. From here head straight, going past the Royal Oxford Hotel (on your right). Cross the bridge. Follow the road as it bears left, then right. Turn left at the T-junction and follow the road (St Giles) up. Somerville college will be on your left. Alternatively get a taxi if feeling puny.
From the bus station to Somerville:
See map (though these directions are slightly different). On the side of the bus station you arrive, there is a passageway through to the market square, go through this and head to the far left side. Here you will spot a sign to a passageway marked `Friar's Entry'. Go through here and turn left at the end. Follow this road (St Giles) up. Somerville college will be on your left.