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[Display omitted] • Pristine C 3 N monolayer shows stronger O 2 physisorption than pristine graphene. • The most preferable O 2 dissociation is an indirect two-step path with chemisorbed O 2. • Two dangling C-O bonds on the para-carbon site is the most stable oxidized structure. • Pristine C 3 N monolayer is more susceptible to oxidation than pristine graphene. • Electronic properties can be altered significantly due to the presence of two chemisorbed O atoms. The oxygen dissociation and the oxidized structure on the pristine C 3 N monolayer in exposure to air are the inevitably critical issues for the C 3 N engineering and surface functionalization yet have not been revealed in detail. Using the first-principles calculations, we have systematically investigated the possible O 2 adsorption sites, various O 2 dissociation pathways and the oxidized structures. It is demonstrated that the pristine C 3 N monolayer shows more O 2 physisorption sites and exhibits stronger O 2 adsorption than the pristine graphene. Among various dissociation pathways, the most preferable one is a two-step process involving an intermediate state with the chemisorbed O 2 and the barrier is lower than that on the pristine graphene, indicating that the pristine C 3 N monolayer is more susceptible to oxidation than the pristine graphene. Furthermore, we found that the most stable oxidized structure is not produced by the most preferable dissociation pathway but generated from a direct dissociation process. These results can be generalized into a wide range of temperatures and pressures using ab initio atomistic thermodynamics. Our findings deepen the understanding of the chemical stability of 2D crystalline carbon nitrides under ambient conditions, and could provide insights into the tailoring of the surface chemical structures via doping and oxidation. [ABSTRACT FROM AUTHOR]