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We present excitation temperatures $T_{ex}$ for the OH 18-cm main lines at1665 and 1667 MHz measured directly in front of the W5 star-forming region,using observations from the Green Bank Telescope and the Very Large Array. Wefind unequivocally that $T_{ex}$ at 1665 MHz is greater than $T_{ex}$ at 1667MHz. Our method exploits variations in the continuum emission from W5, and thefact that the continuum brightness temperatures $T_C$ in this nebula are closeto the excitation temperatures of the OH lines in the foreground gas. Theresult is that an OH line can appear in emission in one location and inabsorption in a neighboring location, and the value of $T_C$ where the profilesswitch from emission to absorption indicates $T_{ex}$. Absolute measurements of$T_{ex}$ for the main lines were subject to greater uncertainty because ofunknown effects of geometry of the OH features. We also employed thetraditional "expected profile" method for comparison with our "continuumbackground" method, and found that the continuum background method providedmore precise results, and was the one to definitively show the $T_{ex}$difference. Our best estimate values are: $T^{65}_{ex} = 6.0 \pm 0.5$ K,$T^{67}_{ex} = 5.1 \pm 0.2$ K, and $T^{65}_{ex} - T^{67}_{ex} = 0.9 \pm 0.5$ K.The $T_{ex}$ values we have measured for the ISM in front of W5 are similar tothose found in the quiescent ISM, indicating that proximity to massivestar-forming regions does not generally result in widespread anomalousexcitation of OH emission.

OH as an Alternate Tracer for Molecular Gas: Excitation Temperatures of the OH 18-cm Main Lines in W5