The gas-phase reactivity of Os(a(5)D(4)) with N2O and O-2 in the temperature range 373-623 K is reported. Osmium atoms were produced by the photodissociation of osmocene and detected by laser-induced fluorescence. The reaction rate constants of the ground a(5)D(4) state with N2O and O-2 are temperature-dependent. The bimolecular rate constants are described in Arrhenius form by k(N2O) = (5.8 +/- 1.4) x 10(-11) exp(-38.1 +/- 1.2 kJ/mol/RT) cm(3) s(-1) and k(O-2) = (2.7 +/- 0.4) x 10(-11) exp(-9.0 +/- 0.5 kJ/mol/RT) cm(3) s(-1) where the uncertainties are +/-2 sigma. The disappearance rates in the presence of both reactants are independent of buffer gas identity (Ar or N-2) and total pressure, indicating a bimolecular abstraction mechanism.