The structural, electronic, and optical properties of oxygen-defective Zn3N2 were studied by means of density functional theory. The geometry optimization result shows that pure Zn3N2 is cubic in structure, which is in agreement with experiment. Our results indicate that Zn3N2 with nitrogen replaced by oxygen is more stable than that with interstitial oxygen and the substitutional oxygen for nitrogen in defective Zn3N2 is responsible for the n-type conduction character. The possible optical transition mechanisms are discussed.