The production of OH in a nanosecond pulsed filamentary discharge generated in pinpin geometry in a HeH 2O mixture is studied by time and spatially resolved laser-induced fluorescence. Apart from the OH density the gas temperature and the electron density are also measured. Depending on the applied voltage the discharge is in a different mode. The maximum electron densities in the low- (1.3kV) and high-density (5kV) modes are 2×10 21m -3 and 7×10 22m -3, respectively. The gas temperature in both modes does not exceed 600K. In the low-density mode the maximum OH density is at the centre of the discharge filament, while in the high-density mode the largest OH density is observed on the edge of the discharge. A chemical model is used to obtain an estimate of the absolute OH density. The chemical model also shows that charge exchange and dissociative recombination can explain the production of OH in the case of the high-density mode.