I was asked this on a weight loss forum, recently.
The answer is, all oils, including olive oil, are 100% fat and will affect your weight loss. So they need to be used sparingly if you want to lose weight.
Olive oil became popular as a result of the Mediterranean Diet which some experts believe is responsible for the low levels of heart disease in Mediterranean countries.
BUT… a recent study in the Journal of the American College of Cardiology into the Mediterranean diet found that olive oil itself may not help reduce the risk of heart disease after all. Olive oil actually slowed down blood flow – a bad sign. But this effect was reduced if plenty of fruit and vegetables or fish had been eaten.
My advice is to use all oils sparingly. Ideally use them in dressings rather than heating them, and of course, eat up your fruit and vegetables!