I am not an expert, but I've read in many places that the omega-3 in flax oil is far inferior, to the point of almost being worthless, compared with fish oil. IIRC the problem is that flax contains omega-3 in the form of ALA, in contrast to fish oil, which contains EPA and DHA -- the forms we actually need. The body can convert ALA to EPA and DHA, but the conversion process is extremely inefficient (something like only 5% get converted).