Preboil Hop Addition – BrewBoard: “Boiling wort is devoid of oxygen as gas solubility in liquids decreases with increasing temperature, but oxidation reactions of oils take place at higher temperatures, making the mash runoff an ideal place to oxidize hop oil compounds. This oxidation makes the compounds more stable, lowering their boiling point (less volatile) while also changing their flavor slightly. The exact reactions I’m sure are pretty complicated. The end result though, is usually damn tasty.”
…and…
Preboil Hop Addition – BrewBoard: “Just to add to this, fats, oils, lipids are usually completely hydrophobic molecules or like membrane lipids are amphipathic (have both a hydrophobic and hydrophilic group) The oxidation of these oils actually makes them soluble and thus adding to the long-term stability of these flavour compounds in beer.”