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.
I don't think you can really transfer money from an Oyster card to another travel card. But if you don't need your Oyster card anymore, you can refund it and use the money then to buy the travel card.