As Globaliser stated, anybody can get an oyster card on the sport at a tube station. The only caveat is that you will have to pay a GBP3 deposit. If you are a visitor going to use your card just one time, this is not efficient. If you travel regularly, or even occasionally to London, you might as well get one. In theory, you can get your deposit back by surrendering your card, but it is not done on the spot (you have to write in, get a cheque back through the post, ...) so not terribly practical unless you live in the UK.