The oxidative half-reaction of copper amine oxidase (CAO) has been studied quantum mechanically using hybrid density functional theory (B3LYP). After the production of H2O2, two different but energetically similar mechanisms leading to the formation of ammonia and TPQ(ox) are suggested. In one of the mechanisms (A), protonation of the O-2 site of the cofactor TPQ occurs prior to water activation whereas in the other (B) the O-2 site of the cofactor remains unprotonated. Both of these mechanisms are divided into four steps, and the last two steps in mechanism B are further divided into two steps. Detailed comparisons are made to experimental findings and suggestions. In particular, the roles of the copper metal center and the critical Asp319 are discussed.