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    The oxidation stability of borophene is deemed as a prerequisite for its broad applications; however, there is a contradiction from experiments as to whether boron atoms in borophenes are active or inert to oxidation. Our detailed density functional theory calculations performed herein indicate that O2 molecules tend to be chemisorbed on supported β12-borophene easily and dissociate into separated atoms by overcoming very low barriers, and spin triplet-to-singlet conversion is not important in the chemisorption process. It is found that O2 molecules prefer to be adsorbed on two hexacoordinated boron atoms and then dissociate and diffuse along filled-hexagon ribbons. A comparison between our calculated core-level binding energies and the experimental X-ray photoelectron spectroscopy, in combination with detailed kinetic analyses, indicates that boron atoms in supported borophenes are active rather than inert to oxidation. This conclusion is further supported by a novel B5O4 sheet model calculation. The results on the oxidation stability and mechanism suggest the protection of borophenes from oxygen is therefore essential for their broad applications.