Thats not true about it becoming a trans-fat as below:
Olive Oil Myth: Olive oil loses its benefits when heated
The Facts: Excessively heating olive oil will evaporate the alcohols and esters which make up its delicate taste and fragrance. Heating olive oil will not change its health aspects, only the flavor. Use a cheaper olive oil which doesn't have much flavor to begin with if you want to fry with it, add a more flavorful olive oil after cooking or at the table.
Olive Oil Myth: Heating a cooking oil will make it saturated or a trans-fatty oil.
The Facts: As far as making a saturated fat, according to Dr. A. Kiritsakis, a world renowned oil chemist in Athens, (Book - OLIVE OIL FROM THE TREE TO THE TABLE -Second edition 1998), all oils will oxidize and hydrogenate to a tiny degree if repeatedly heated to very high temperatures such as is done in commercial frying operations. Olive pomace oil and virgin olive oil are both highly monounsaturated oils and therefore resistant to oxidation and hydrogenation. Studies have shown oxidation and hydrogenation occurs to a lesser degree in olive oil than in other oils. But in any case, the amount of hydrogenation is miniscule and no home cook would ever experience this problem.
The large refinery-like factories which take unsaturated vegetable oil and turn it into margarine or vegetable lard do so by bubbling hydrogen gas through 250 to 400 degree hot vegetable oil in the presence of a metal catalyst, usually nickel or platinum. The process can take several hours. You cannot make a saturated product like margarine at home by heating olive oil or any other vegetable oil in a pan. We don't know where this weird notion has come from. For more see our olive oil chemistry page
Changing a cis-fat to a trans-fat does not occur on a home stove.
Olive Oil Myth: Cooking in olive oil diminishes the nutritional value of the food.
Olive Oil Fact: Heating food will break down its nutritional value. High heat such as frying is worse than moderate heat such as steaming, which is worse than eating vegetables raw. It is not the cooking oil per se, but the high heat of frying. I am not aware of any edible cooking oil which of itself diminishes the nutritional value of the food cooked in it. Most nutritionists recommend lightly steaming vegetables or eating them raw. A touch of a flavorsome olive oil added at the table will add taste and healthful anti-oxidants. Such is the "Mediterranean diet" which has been shown to help prevent coronary disease and have other health benefits.