Dilute magnetic semiconductors have auracted much attention because of their potential applications in spintronics. In this paper, the effects of oxygen vacancy on total energy and magnetism in Co-doped TiO2 are investigated using the density functional theory. The energy of the system with a shorter Co-Co distance is higher than that with larger Co-Co distance after introducing oxygen vacancy. Oxygen vacancy trends to congregate around the Co cations. Moreover, the strength of exchange couple reduces in the system with a shorter Co-Co distance after introducing oxygen vacancy. For the system with a larger Co-Co distance, the exchange couple between two Co impurities is anti-ferromagnetic if oxygen vacancy is located at the basal site of octahedron containing Co, and ferromagnetic if oxygen vacancy is located at the apical site of octahedron containing Co.