The formation of electronically excited atomic oxygen was studied behind reflected shock waves using cavity-enhanced absorption spectroscopy. Mixtures of 1% O-2-Ar were shock-heated to 5400-7500 K, and two distributed-feedback diode lasers near 777.2 and 844.6 nm were used to measure time-resolved populations of atomic oxygen's S-5 degrees and S-3 degrees electronic states, respectively. Measurements were compared with simulated population time histories obtained using two different kinetic models that accounted for thermal nonequilibrium effects: (1) a multitemperature model and (2) a reduced collisional-radiative model. The former assumed a Boltzmann distribution of electronic energy, whereas the latter allowed for non-Boltzmann populations by treating the probed electronic states as pseudospecies and accounting for dominant electronic excitation/de-excitation processes. The effects of heavy-particle collisions were investigated and found to play a major role in the kinetics of 0 atom electronic excitation at the conditions studied. For the first time, rate constants (k(M)) for O atom electronic excitation from the ground state (P-3) due to collisions with argon atoms were directly inferred using the reduced collisional-radiative model, k(M)(P-3 -> S-5 degrees) = 7.8 X 10(-17)T(0.5) exp(-1.061 X 10(5)K/T) +/- 25% cm(3) s(-1) and k(M)(3P -> S-3 degrees) = 2.5 X 10(-17)T(0.5) exp(-1.105 X 10(5)K/T) +/- 25% cm(3) s(-1).