Updated March 29, 2021
Chia, Flax and Walnut are all good sources of omega-3 oils, the plant version called alpha linolenic acid (click on diagram to enlarge).
Omega-3 oils convey fluidity to surfaces such as cell membranes. They maintain their flexibility in cold temperatures, unlike other oils that will become a butter or wax when cold (such as coconut oil). That is why their one of their greatest sources is cold-water fish.
However, this superiority in the cold render them sensitive to heat and oxidation (their double bonds convey flexibility and vulnerable). When an oil is oxidized by heat, light, or other means, it is called rancid. The smell is more bitter/pungent and the color is darker. Smelling a slightly open bag or bottle of walnuts or flax oil will be an indication of rancidity. Flax oxidizes at 225 F, so slow cooking at low temperatures can be OK. Adding anti-oxidants such as vitamin E can protect the oil in a recipe. Cooking omega-3 fish like salmon remains controvercial. Smoking, pickling and salting are claimed to be ways of preserving these proteins without damaging the oils (need to do some fact-checking here). Most agree that Omega-3 Foods seeds begin to oxidize after grinding, so they should be stored in a cold, dark place (the refrigerator or freezer) unground for up to a year (label your bottle!). After grinding keep them in the refrigerator for up to a week. Needless to say, it is useless to buy pre-ground flax!