We present a first-principles study on the electronic properties of TiO containing both dopants and defects on the basis of density-functional theory. We show that the formation energies of defects can be reduced by up to 80% in N-doped TiO (N substitutes O), as compared to those in perfect TiO, but the H-doping (H substitutes O) has less effect on the defect formation. We predict that dopant-interstitial Ti cluster plays an important role in the narrowing of band gap of N-doped anatase and brookite TiO, and H-doped rutile TiO. Importantly, the defect bands within the band gaps can enhance the visible-light absorption and improve the photocatalytic performance of the systems due to the reduced gap between the bands. The dopant-defect cluster in the doped TiO may be responsible for the observed visible-light absorption in experiments. The present study provides a new route to enhance the performance of photocatalyst.