I will be in London for three days next week. I'll be walking and taking the Metro system so I'm wondering if I should be the Oyster card and if so, where do I purchase it?
It's not the Metro, it's called the Underground or the Tube. Yes you should get an Oyster card and you can purchase it at any tube station.
You pay a deposit of 3 british pounds (maybe 3.50?), but you can get it refunded if you return the card to an Underground ticket counter (they also have boxes where you can deposit it as a charitable contribution).
Yes, you should get the Oyster card..and as Frank II said it's the Underground or Tube not the Metro.
By all means get an oyster card. It is also good for their bus system, which is safe, clean, and very efficient.I was just in London a couple of weeks ago, , got a 2 zone oystercard for 3 days...and it made getting around SO easy....Even worked on catching a train back from Greenwich. We must have used it at least 12 times for the Tube, and for at least 8-10 busses....and we never had to top up the card!
We were there for 4 days, it's great and easy to use. There is a cap for how much you will pay each day. Also, very easy to turn in at the end to get your deposit back.
It is very easy to get more money added to the Oyster card should you need it. It is so easy to use that I wouldn't consider any other option. Just be sure to keep it handy but in a safe place. When we purchased the cards at Heathrow, we were given little plastic carrying cases which were helpful, but we still lost one.