Don't let anyone tell you that flaxseed oil or another kind of plant-based EFA source works as well. It doesn't. Your cat is a carnivore and cannot derive all that she needs EFA-wise from PLANT sources.
Geeky details: There are many ways in which cats function differently biochemically from dogs. Cats, for example, cannot make their own Arachidonic Acid even when there is linoleic acid present. A cat's "chemical factory" (liver), contains no delta-6 desaturase enzyme to make the conversion of linoleic acid to Arachidonic acid. And there is little or no Arachidonic acid in plant matter. A dog, on the other hand, if consuming the proper fats, can make Arachidonic acid.
A cat would normally get ample EFAs from eating the brains and eyes of her prey. So unless you're feeding brains and eyes, don't count on flaxseed to get EFAs into your cat.