The kinetics and mechanisms of the reactions of cyanomidyl radical (HNCN) with oxygen atoms and molecules have been investigated by ab initio calculations with rate constant prediction. The doublet and quartet state potential energy surfaces (PESs) of the two reactions have been calculated by single-point calculations at the CCSD(T)/6-311+G(3df, 2p) level based on geometries optimized at the CCSD/6-311++G(d, p) level. The rate constants for various product channels of the two reactions in the temperature range of 300-3000 K are predicted by variational transition state and RRKM theories. The predicted total rate constants of the O(3P) + HNCN reaction at 760 Torr Ar pressure can be represented by the expressions ktotal (O+ HNCN) = 3.12 × 10-10 × T-0.05 exp (-37/T) cm3 molecule-1 s-1 at T= 300-3000 K. The branching ratios of primary channels of the O(3P) + HNCN are predicted: k1 for producing the NO + CNH accounts for 0.72-0.64, k2 + k9 for producing the 3NH + NCO accounts for 0.27-0.32, and k6 for producing the CN + HNO accounts for 0.01-0.07 in the temperature range studied. Meanwhile, the predicted total rate constants of the O2 + HNCN reaction at 760 Torr Ar pressure can be represented by the expression, ktotal(O2 + HNCN) = 2.10 × 10-16 × T1.28exp (-12200/T) cm3 molecule-1 s -1 at T= 300-3000 K. The predicted branching ratio for k 11+k13 producing HO2 + 3NCN as the primary products accounts for 0.98-1.00 in the temperature range studied.