The potential of using p-doped nanocrystalline diamond as the anode for organic solar cells, because of its outstanding photostability and well-matched energetics with organic dyes, is demonstrated. The interface dipole and open-circuit potential can be tuned by varying the surface termination on diamond. Oxygenated nanocrystalline diamond (O-NCD) exhibits the best photocurrent conversion among all the surface-treated electrodes studied in this work because of its large open-circuit potential. The good energy alignment of the valence band of O-NCD with the HOMO of poly(3-hexylthiophene), as well as its p-doped characteristics, suggest that O-NCD can replace the hole transport layer, such as PEDOT:PSS, needed for efficient performance on indium tin oxide (ITO) electrodes. If the sheet resistance and optical transparency on NCD can be further optimized, chemical-vapor-deposited diamond electrodes may offer a viable alternative to ITO and fluorinated tin oxide (FTO).