Fish oil is a popular dietary supplement that is derived from the fatty tissues of certain types of fish. The two most common sources of fish oil are COD and haddock, both of which are three-letter abbreviations: CAD and COD. COD is the most widely used source of fish oil, as it is rich in omega-3 fatty acids, which are beneficial for human health. Haddock is also a good source of omega-3 fatty acids, but it is not as widely used as COD.