Oyster card is your cheapest travel card option, if you are going to/from work everyday then a period pass (7 days or a month) will work out cheaper.

An Oyster card not only holds your period pass but allows you to maintain a credit balance too. So if you buy zones 1 & 2 but one evening, against your better judgement you decide to go to Tuffnell Park, or East Ham, then the zone extension will be debited from your prepaid balance.

If you don't have a prepaid balance then you can buy the extension before you enter the network