Flax oil is more for overalll health, not building mass. People's fat ratio's are sort of screwed, we get too much Omega-6 in comparison to Omega 3. Flax is an Omega-3 (I think), which helps balance the ratio, and improves health. I think the optimum ratio is 4 grams of Omega 6 to every gram of Omega 3. Or it might be 6 to 1? But I've read, most people eat like 20 times as much Omega 6 as omega 3.
Fish fat (like Salmon) is another good source of Omega-3.