Practical applications of two-dimensional (2D)black phosphorus (BP) are limited by its fast degradationunder ambient conditions, for which many differentmechanisms have been proposed; however, an atomic levelunderstanding of the degradation process is still hindered bythe absence of bottom-up methods for the growth of large-scale few-layer black phosphorus. Recent experimental successin the fabrication of single-layer blue phosphorus provides amodel system to probe the oxidation mechanism of two-dimensional (2D) phosphorene down to single-layer thick-nesses. Here, we report an atomic-scale investigation of theinteraction between molecular oxygen and blue phosphorus.The atomic structure of blue phosphorus and the localbinding sites of oxygen have been precisely identified using qPlus-based noncontact atomic force microscopy. A combination oflow-temperature scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements reveal a thermallyreversible oxidation process of blue phosphorus in a pure oxygen atmosphere. Our study clearly demonstrates the essential roleof oxygen in the initial oxidation process, and it sheds further light on the fundamental pathways of the degradation mechanism.