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OH is an important molecule in the H2O chemistry and the cooling budget ofstar-forming regions. The goal of the Herschel key program `Water inStar-forming regions with Herschel' (WISH) is to study H2O and related speciesduring protostellar evolution. Our aim in this letter is to assess the originof the OH emission from star-forming regions and constrain the properties ofthe emitting gas. High-resolution observations of the OH 2Pi1/2 J = 3/2-1/2triplet at 1837.8 GHz (163.1 micron) towards the high-mass star-forming regionW3 IRS 5 with the Heterodyne Instrument for the Far-Infrared (HIFI) on Herschelreveal the first hyperfine velocity-resolved OH far-infrared spectrum of astar-forming region. The line profile of the OH emission shows two components:a narrow component (FWHM approx. 4-5 km/s) with partially resolved hyperfinestructure resides on top of a broad (FWHM approx. 30 km/s) component. Thenarrow emission agrees well with results from radiative transfer calculationsof a spherical envelope model for W3 IRS 5 with a constant OH abundance ofapprox. 8e-9. Comparison with H2O yields OH/H2O abundance ratios of around 1e-3for T > 100 K and around unity for T < 100K, consistent with the currentpicture of the dense cloud chemistry with freeze-out and photodesorption. Thebroad component is attributed to outflow emission. An abundance ratio of OH/H2O> 0.028 in the outflow is derived from comparison with results of water linemodeling. This ratio can be explained by a fast J-type shock or a slowerUV-irradiated C-type shock.

First hyperfine resolved far-infrared OH spectrum from a star-forming region