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The energy to desorb atomic oxygen from an interstellar dust grain surface,$E_{\rm des}$, is an important controlling parameter in gas-grain models; itsvalue impacts the temperature range over which oxygen resides on a dust grain.However, no prior measurement has been done of the desorption energy. We reportthe first direct measurement of $E_{\rm des}$ for atomic oxygen from dust grainanalogs. The values of $E_{\rm des}$ are $1660\pm 60$~K and $1850\pm 90$~K forporous amorphous water ice and for a bare amorphous silicate film,respectively, or about twice the value previously adopted in simulations of thechemical evolution of a cloud. We use the new values to study oxygen chemistryas a function of depth in a molecular cloud. For $n=10^4$ cm$^{-3}$ and$G_0$=10$^2$ ($G_0$=1 is the average local interstellar radiation field), themain result of the adoption of the higher oxygen binding energy is that H$_2$Ocan form on grains at lower visual extinction $A_{\rm V}$, closer to the cloudsurface. A higher binding energy of O results in more formation of OH andH$_2$O on grains, which are subsequently desorbed by FUV radiation, withconsequences for gas-phase chemistry. For higher values of $n$ and $G_0$, thehigher binding energy can lead to a large increase in the column of H$_2$O buta decrease in the column of O$_2$.

A New Determination of the Binding Energy of Atomic Oxygen on Dust Grain Surfaces: Experimental Results and Simulations