本人會於倫敦停留一星期，請問買Oyster card定7 day Travel pass好呢？

If you travek in Zone 1 to zone 2 Oyster card will absolutely cheaper because it have a max price for every day, if you use the fares over that price the card can still use but no more deduction on the credit.
I have calculated, cheaper that 1 day pass, or 7 day pass

max price everyday is same as 1 day pass... 7 day pass is cheaper than 7 * 1 day pass.