In a previous work, erbium doped AlN thin films were deposited by r.f. magnetron in Ar+N-2 discharge. Both the erbium content of the films and post-deposition thermal annealing were optimized in order to maximize the Er3+ room temperature photoluminescence (PL) at 1.55 mum. Since oxygen is known to enhance the PL emission of erbium doped semiconductor thin films, in this work AlN(Er) films were intentionally doped during growth by r.f. magnetron sputtering with oxygen contents between 6.2 and 58.6%. Thermal annealing of the deposited films was carried out in order to enhance the PL emission. The structure of both as deposited and annealed films was characterized by X-ray diffraction. Finally, the 1.55 pin Er3+ PL emission was recorded as a function of the oxygen content in the films. The maximum PL intensity is achieved for films with an oxygen to erbium ratio between 2.6:1 and 6.8:1 after annealing for one hour at 1075 K.