You can't convert all recipes from solid shortening (butter/margarine) to liquid shortening (olive oil/vegetable oil). For instance, a cake frosting must stay solid at room temperature, so a quick frosting made with butter and powdered sugar would work whereas olive oil and powdered sugar wouldn't.

Then there is the taste. A mild-tasting late harvest olive oil would work great in most cake, pastry recipes and your oatmeal cookies. Olive oil can kick up the flavor in brownies and other strong foods. The flavor in olive oil is blunted with cooking. Uncooked confections such as the cake frosting would taste more than a bit unusual if made with a strong olive oil.

For most main course dishes where margarine or butter is being used for frying or sautéing, olive oil could be readily substituted. In olive oil-producing countries the flavors of olive oil and butter/margarine are used to enhance each other in some recipes.