Olive oil is best added uncooked, that is don't cook or use it in cooking as the heat will decrease it's nutritional value. I'm not quite familiar with which oil (flaxseed, fish, sesame, vegetable, soy, peanut, etc) is most beneficial for a lo at this age but overall the oil is used to fatten the lo up. The most important oils for a lo are DHA (in bm) and ARA (body converts this from regular fats) which is for infant brain growth and eye and nerve development.
Overall, don't worry about adding any extra fats into her diet unless her dr suggests it. Los comes in different sizes. As long as she is energetic, active and developing "normally" then body size doesn't matter.
