For February all donations made to Oxfam through Paypal are completely free to Oxfam, meaning 100% of your donation goes directly to them :)
All other sites (e.g. JustGiving, Virgin Money Giving etc.) charge the charity a fee to process the donation (see http://www.moneysavingexpert.com/family/charity-fundraising-sites
for a breakdown) so donating via Paypal will save Oxfam having to use some of your money to pay these.
In effect you're saving the charity 2-3%, not Earth-shattering but still worth doing.
The option to gift aid your donation is still available too.