Structural and electronic properties of bulk GaN with nitrogen vacancy($V_N$), oxygen substitution on nitrogen site($O_N$), and complex of nitrogen vacancy and oxygen substitution on nitrogen site($V_N-O_N$) were investigated using the first principle calculations. It was found that stability of defect formation is dependent on the epilayer growth conditions. The complex of $V_N-O_N$ is energetically the most favorable state in a condition of Ga-rich, however, oxygen substitution in nitrogen site is the most favorable state in N-rich condition. The electronic property of complex with negative charge states at $Gamma$ point was changed from semiconductor to metal. On the contrary, the properties of nitrogen vacancy except for neutral charge state have shown the semiconductor characteristics at $Gamma$ point. In the oxygen substitution on nitrogen site, the energy differences between conduction band minimum and Fermi level were smaller than that of defect-free GaN.