Diamond thin films have been grown by an electron-cyclotron-resonance (ECR) microwave plasma chemical vapor deposition (CVD) apparatus under the addition of oxygen to the host ambient gas of a mixture of CO and H2 gas. Although the amount of oxygen was very minute (0-6% vol. fraction), significant changes due to the oxygen were brought about in the growth properties and the film crystallinity. The Raman spectroscopy revealed a dramatically improved film crystallinity and the film was dominated by the grain growth, resulting in the faceted morphology. With increase in the oxygen up to 4.55%, the visible cathodeluminescence (CL) intensity grew by more than ten times. However, the CL spectrum indicated an appearance of one or two extra recombination centers depending on the amount of oxygen.