If you look up fatty acids and cats, it usually recommends fish oil over flaxseed oil. You have to look at how the cat processes the oil and it seems fish oils are better.

The most commonly used fatty acid supplements are fish and flaxseed oils, both of which contain omega-3s. Flaxseed oil actually has more omega-3s than fish oil, but in an inactive form; research studies have therefore indicated that the latter is most beneficial for both people and animals. For this reason, fish oil is generally recommended as the omega-3 fatty acid supplement of choice.
You can read more about it here: Animal Wellness