  The reaction between atomic oxygen and molecular hydrogen is an important one
in astrochemistry as it regulates the abundance of the hydroxyl radical and
serves to open the chemistry of oxygen in diverse astronomical environments.
However, the existence of a high activation barrier in the reaction with ground
state oxygen atoms limits its efficiency in cold gas. In this study we
calculate the dependence of the reaction rate coefficient on the rotational and
vibrational state of H$_2$ and evaluate the impact on the abundance of OH in
interstellar regions strongly irradiated by far-UV photons, where H2 can be
efficiently pumped to excited vibrational states. We use a recently calculated
potential energy surface and carry out time-independent quantum mechanical
scattering calculations to compute rate coefficients for the reaction O(3P) +
H2(v,j) -> OH + H, with H2 in vibrational states v = 0-7 and rotational states
j = 0-10. We find that the reaction becomes significantly faster with
increasing vibrational quantum number of H2, although even for high vibrational
states of H2 (v = 4-5) for which the reaction is barrierless, the rate
coefficient does not strictly attain the collision limit and still maintains a
positive dependence with temperature. We implemented the calculated
state-specific rate coefficients in the Meudon PDR code to model the Orion Bar
PDR and evaluate the impact on the abundance of the OH radical. We find the
fractional abundance of OH is enhanced by up to one order of magnitude in
regions of the cloud corresponding to Av = 1.3-2.3, compared to the use of a
thermal rate coefficient for O + H2, although the impact on the column density
of OH is modest, of about 60 %. The calculated rate coefficients will be useful
to model and interpret JWST observations of OH in strongly UV-illuminated
environments.
