ANSWER
Oxygen exposure is a catalyst for oxidation. Volatile compounds are formed during lipid oxidation reaction between oxygen and DHA in oil. Hydroperoxides are formed and then broke down into smaller compounds known as volatiles (such as aldehydes and ketones) which are responsible for the off-flavours and rancid odours.
Below are studies:
Chemical-Measures-of-Fish-Oil-Quality-Oxidation-Products-and-Sensory-Correlation
Effects-of-Processing-on-the-Oxidative-Stability-of-Docosahexaenoic-Acid-Oil
EFSA-Scientific-Opinion-on-Fish-Oil-for-Human-Consumption-Food-Hygience-including-Rancidity
