Okay. I can give you a proper answer to this.
If you are flying with Ryanair, we do sell tickets on board, train tickets and also terra vision bus tickets.
Trains:
Cheaper if you get it on board. Takes 45mins to Liverpool street.
In case you want to go to Oxford street, you need to catch the tube from Liverpool street stn [Central line, the red one to Oxford Circus.]
If you chose to go by bus:
You can also buy tickets on board, true it is the same price like at the airport or on the bus, but you can atleast save time as you don't have to stand in the queue for hours.
Bus tickets are from £9, but since you would need one to Victoria is £12 [if I am not mistaken] and from Victoria you need to catch the tube [Victoria Line, light blue. But since this is the only one, you can't miss it] to Oxford Circus
Train is quicker and bus is cheaper.
A return ticket from Liverpool street is £17
Further information about traveling in London, in case you needed
http://www.tfl.gov.uk