oxidation doesn't always mean oxygen though. oxidation means a loss of electrons from any element to another.
the element that loses the electron is oxidised, the one that gains an electron is reduced.
I try to think of it as seats around table at a function and some tables at a function are more attractive than others.
Oxygen has 8 seats and 2 seats empty and is very attractive. Hydrogen has only one seat thats full in its natural state.
All elements are looking to fill there available seats (become noble).
the analogy doesn't completely work when you read more into different types of chemical bonds but it helps to remind me of how many electrons a element needs to become noble (fill all seats) If oxygen binds to 2 hydrogen atoms its seats are full forming H2O, but you never see H30 as there is only 8 seats at the oxygen table. im waffling.
in your beers case its probably oxygen reacting with the beer or a settling of tiny fine particles that capture light, resulting in the perception of a darker beer (even bright beer isn't completely transparent)