The prices for travelling with either an Oyster card or by buying a Travel card/individual single tickets are exactly the same. So you don’t save any money by having an Oyster card. You can see the prices at this page:
http://www.tfl.gov.uk/tickets/14416.aspx
For only two days it seems like a waste of time to me to obtain an Oyster card. You can get an Oyster card for visitors from the Transport for London shop that is prepaid (http://visitorshop.tfl.gov.uk/home.html), but you pay a £3 activation fee, and it doesn’t make it any faster than using a regular paper ticket.
You don’t save any money, assuming you make enough single journeys in one day that you reach the price cap (when you hit the price cap with an Oyster card it is just the same as if you bought a Travelcard, you get unlimited free travel for the rest of the day).
It makes a lot more sense to just buy a travel card every day for Zones 1-2 (you probably won’t go outside of those zones). Then you get unlimited travel, and you can use them on the bus too.
Credit:stackoverflow.com‘