The authors demonstrate a sample method to achieve local tuning of optical modes in GaAs photonic crystal nanocavities by continuous wave laser-assisted oxidation in air atmosphere. By irradiation with a focused laser beam at power levels of a few tells of milliwatts, photonic crystal nanocavity modes shift to shorter wavelengths by Lip to 2.5 nm. The mode shifts call be controlled either by varying the laser power or by iterating laser-assisted oxidation steps and are well explained by finite-element-method and finite-difference time-domain simulations. This method provides a simple route to achieve fine spectral tuning of individual nanocavities for photonic devices. (C) 2009 American Institute of Physics [doi: 10 1063/1.3262961]