Not all oils are the same, with many being poor quality.
Fish do not produce omega-3. It's produced through photosynthesis and is then eaten by lots of small sea animals.
A fish will then eat lots of them in turn and thus concentrate the omega-3 within their flesh.
With this in mind, you can understand that wild-harvested fish will be better than farmed fish (which are fed pellets often boosted with antibiotics).
We also want a short-lived fish as research shows that the longer an animal lives in the ocean the greater the concentration of pollutants.
So mulies, for instance are great because they are a short-lived oily fish that is wild harvested.
The other problem I see is that, when a manufacturer concentrates the omega-3 in fish oil, it has to take out some of the other omega oils (omega-6 and 9).
I try not to plug over-the-counter supplement brands in this column but I think it will help you to know that my preference in this area is BioCeuticals EPA/DHA.
As for oils versus capsules, in a capsule the oils are protected from oxidisation by the airtight capsule.
Further, as a guide, krill oil is best for treating high cholesterol, fish oil for hormonal, skin and inflammation issues.