I got an Oyster card from my family which is topped up with 42 pounds. Actually I was thinking of buying a 7-day travel card once I arrive in London. Can I buy or top up 7-day travel card using that credit already charged on the Oyster card?
If you have 10 pounds or less on your Oyster card, if you cancel it (at a vending machine in the Underground) you'll get the credit and the 5-pound deposit fee refunded in cash (so up to 15 pounds).
You can then use that money towards buying a new Oyster card (at the same vending machine) with a 7-day Travelcard on it.