Laser annealing experiments on commercially available phase pure tenorite (CuO) nanoparticles (NPs) were performed in air and nitrogen atmosphere to improve the structural and electronic properties, with respect to their suitability for photovoltaic applications. The particles exhibit size variations from about 30 nm to 100 nm. The influence of the thermal treatment is investigated by photoluminescence (PL) and Raman spectroscopy. Annealing of the particles in air by a laser treatment improved the material quality by defect reduction. Additional laser annealing in N2 atmosphere leads to a phase transition of the NPs from tenorite to cuprite (Cu2O). Due to the low partial oxygen pressure, the transition is initiated at about 1/3 of the maximum laser power used for the series in air, which is indicated by a drastic increase of the band edge emission from Cu2O. However, annealing with higher laser power leads to strong defect luminescence, which originates from copper and oxygen vacancies. A weak remaining tenorite band edge emission shows an incomplete phase transition.