Near-surface laser-induced fluorescence (LIF) measurements of OH are performed in the post-flame region of low pressure hydrogen/oxygen flames in stagnation flows above catalytic and non-catalytic surfaces. Measurements of the normalized gradient of the OH number density near the surface are shown to be sensitive to changes in surface reaction rates, and thus can be used as a test of surface chemical mechanisms. In addition, such measurements do not require an absolute calibration, which increases the accuracy of the comparison. A comparison of the observed results for varying substrate temperature, system pressure, and flame equivalence ratio suggests that the recombination probability of OH on platinum is overpredicted in general by current surface models, and that the dependence of recombination on pressure is stronger than predicted. The qualitative trends for OH recombination with temperature and equivalence ratio are similar to predicted trends.