  Water photodissociation in the 114 - 144 nm UV range forms excited OH which
emits at mid-infrared wavelengths via highly excited rotational lines. These
lines have only been detected with Spitzer in several proto-planetary disks and
shocks. Previous studies have shown they are a unique diagnostic for water
photodissociation. Thanks to its high sensitivity and angular resolution, the
James Webb Space Telescope (JWST) could be able to detect them in other
environments such as interstellar Photo-Dissociation Regions (PDRs). In order
to predict the emerging spectrum of OH, we use the Meudon PDR Code to compute
the thermal and chemical structure of PDRs. The influence of thermal pressure
($P_{\rm th}/k$ = $n_{\rm H} T_{\rm K}$) and UV field strength on the
integrated intensities, as well as their detectability with the JWST are
studied in details. OH mid-IR emission is predicted to originate very close to
the H$^0$/H$_2$ transition and is directly proportional to the column density
of water photodissociated in that layer. Because neutral gas-phase formation of
water requires relatively high temperatures ($T_{\rm K} \gtrsim 300~$K), the
resulting OH mid-IR lines are primarily correlated with the temperature at this
position, and are therefore brighter in regions with high pressure. This
implies that these lines are predicted to be only detectable in strongly
irradiated PDRs ($G_0^{\rm incident}$ $>$ 10$^3$) with high thermal pressure
($P_{\rm th}/k$ $\gtrsim$ 5$\times$10$^7$ K cm$^{-3}$). In the latter case, OH
mid-IR lines are less dependent on the strength of the incident UV field. The
detection in PDRs like the Orion bar, which should be possible, is also
investigated. To conclude, OH mid-IR lines observable by JWST are a promising
diagnostics for dense and strongly irradiated PDRs.
