A pulsed laser photolysis-discharge flow-photoionization mass spectrometer system has been employed to investigate the kinetics of the reactions NH2 + O(P-3) (atomic oxygen) and HNO + O(P-3) over the temperature range 242-473 K in 2-7.5 Torr of He. O(P-3) was generated in a microwave discharge of an oxygen-helium mixture, while NH2 was produced by 193 nm ArF laser photolysis of NH3. HNO was formed as a reaction product of NH2 + O. Both NH2 and HNO were photoionized by an Ar resonance lamp and detected as parent ions in a quadrupole mass spectrometer. From the analysis of the time profiles of ion signals for various O-atom concentrations, the overall rate constants are best represented by the pressure and temperature-independent values of k(1) = (1.2 +/- 0.3) x 10(-10) and k(2) = (3.8 +/-1.3) x 10(-11) in the units of cm(3) molecule(-1) s(-1) for the NH2 + O and HNO + O reactions, respectively. The value of k(1) is 30-80% larger than values reported previously.