There's
this but it's not an Oyster card, it's just a travel card that's good for the week and costs £27.60 for Zones 1 and 2 (which is prolly all you'll want?)
I thought you only put cash on an Oyster card and it deducts the relevant fares per use up to a daily max? If you're sticking to Zones 1 and 2, and assuming you'll be taking at least 2 journeys per day within those, better value to get a travel card I would think.
I could be wrong though, I'm not local and it's been a while since I've been in London.