Until now, reactions between methane photolysis products (CH3 center dot, CH2) and active N atom or reactive NO radical are proposed as routes of HCN formation in the prebiotic Earth. Scientists think that the reducing atmosphere of primitive Earth was made of H-2, He, N-2, NO, CH4, H2O, CO2, etc., and there was no molecular oxygen. However, it has been evident from experiments that the vacuum ultraviolet (VUV) photolysis of CO2 can produce atomic oxygen. Therefore, it can be presumed that atomic oxygen was likely present in early Earth's atmosphere. Was there any impact of atomic oxygen in production of early atmospheric HCN for the emergence of life? To hunt for the answer, we have employed computational methods to study the mechanism and kinetics of CH3NO + O(D-1) and CH2NO center dot + O(P-3) addition reactions. Current study suggests that the addition of O(D-1) into nitrosomethane (CH3NO) and the addition of O(P-3) into nitrosomethylene radical (CH2NO center dot) can efficiently produce HCN through an effectively barrierless pathway. At STP, Bartis-Widom phenomenological loss rate coefficients of O(D-1) and O(P-3) are obtained as 2.47 x 10(-12) and 4.67 x 10(-11) cm(3) molecule(-1) s(-1), respectively. We propose that addition reactions of atomic oxygen with CH3NO and CH2NO center dot might act as a potential source for early atmospheric HCN.