I use olive oil all the time and have done for years- it shouldn't taste rancid. Even if it's within it's date and sealed, exposure to direct sunlight can cause it to go off. It should be kept in a dark place, and this doesn't always happen in the shop, transport etc. If yours is 'off', this is probably why. I agree, take it back and exchange it. Don't use it, rancid oil is mildly toxic, I believe.