My husband and I are planning a trip in June. We will be staying with a relative who is currently living in London. I just found out that another family member left behind their oyster cards for the next visitor to use. Are they reusable in this way? Also I know there is also the travel card option. If we put money on these oyster cards do we still have the option of adding the travel card benefits? Or if would we have to purchase new cards?
You can definitely reuse the oyster card-there's no id on it. My family passes around our oyster cards with no trouble.
Yes, Oyster cards are "just" an electronic wallet, anybody can use them (but two different people cannot use the same card at the same time. as you have to "tap" in and "tap out).
Whatever credit was left on the card will still be useable. Just return the favour by leaving the card with your relative for the next visitor.
Putting a travel card on an Oyster card is, IMHO, not worth it for a tourist.
Official sit for public transport -- fares, package deals, Oyster functions, interactive trip planning.
Because there's a daily cap on the Oyster Card, you probably won't save much by loading a travel card. If it turns out that you do a lot of walking and tend to spend a lot of time at each sight, you might actually lose a bit on the travel card. It's not going to make a lot of difference either way, I don't think.
What might matter is whether you are interested in a number of the 2-for-1 deals that you qualify for if you buy a paper travel card at a rail station. The devil is in the details, because the list of participating sights is rather long, but so many of London's museums are free. And they're quite large, so if you are heavy consumers of museums, you might have very little time for those 2-for-1 deals even if they interest you. One of the deals is for the Churchill War Rooms, currently one of London's busiest sights and a place for which you'll want to buy your tickets in advance. The 2-for-1 deal requires you to stand in line and buy your tickets at the ticket booth. I've decided that time is more valuable than the potential savings, so even though my travelmate and I both want to go to the War Rooms, we will not be taking advantage of any 2-for-1 offer there.
If the donor oyster cards are the "visitor" type, you can't load travelcards onto them anyway, you can only do that on normal oysters.