Most reactions in solution are between TWO REACTANTS and thus are Second Order. However, water and oxygen are present in unlimited supply and their concentration remains constant throughout - reaction appears to be First order.
Determination of Reaction Order for the main Chemical Degradation Reactions
D   + H2 O ® hydrolysis products
-  d [ D ] / dt = k2 [ D ] [H2 O ] » k1 [ D ]
where    k1 = k2 [H2 O ]
D    + O2 ® oxidation  products
-  d [ D ] / dt = k2’ [ D ] [O2 ] » k1’ [ D ]
where     k1’ = k2’ [O2 ]