In this work, the effect of microwave annealing of tungsten oxide films, which are widely used to enhance hole extraction in organic optoelectronic devices such as organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs), on the nanomorhology and optical properties of bulk heterojuction (BHJ) OPVs, is demonstrated. It is found that a short microwave exposure of under-stoichiometric oxide film enhances the crystallinity/ordering of poly(3-hexylthiophene):[6,6]-phenyl-C71-butyric acid methyl ester (P3HT:PC 71 BM) blends when coated on microwave annealed tungsten oxide films, as revealed from X-ray diffraction and UV-Vis absorption measurements. The performance of OPVs using microwave annealed tungsten oxides and based on a P3HT:PC 71 BM blend as the photoactive layer reached values of 3.97%, an increase of about 53% compared with the device using the under-stoichiometric tungsten oxide hole extraction layer not subjected to microwave annealing.