The biggest problem with olive oil is in the imported stuff. It's exported from various countries in bulk, then sold by brokers who up their profits by mixing in cheaper oils. Some of Costco's olive oil is imported from Italy, and some comes from California. The one I bought from Amazon (nearest Costco is 300+ miles away) stated that it was imported, so I guess I got screwed. But the point is that if they label it from California, it's most likely pure.
"Right is right, even if no one is doing it; wrong is wrong, even if everyone is doing it." - St. Augustine
I use fermented cod liver oil by green pastures , high quality.
Tinned fish always in spring water. Nothing else .
There's several shops In markets in london that sell them like shepards bush etc.
Wild Planet canned fish has no added oils, but it's expensive. Next best is olive oil.
But best if to eat fresh fish daily.
Also "Fish Oil" is not the same as "Omega-3". But I'd guess that fish oil from anchovies, sardines, mackrel, etc. are pretty close.
"When the search for truth is confused with political advocacy, the pursuit of knowledge is reduced to the quest for power." - Alston Chase