An atmospheric-pressure surface dielectric barrier discharge in nitrogen–oxygen mixtures is studied under microsecond (ls) and nanosecond (ns) pulsed excitation. Using intensified charge-coupled device imaging and phase-resolved optical emission spectroscopy, supported by two- dimensional plasma–fluid simulations, the influence of oxygen content, pressure, and pulse characteristics on streamer formation and propagation is systematically examined. The ls-pulsed discharge exhibits strongly stochastic, filamentary behavior, whereas the ns-pulsed discharge is more reproducible and suitable for phase-resolved analysis, while still exhibiting some localized filamentation, particularly for positive streamers. Lowering the oxygen content at fixed pressure mainly enhances positive-streamer propagation, while negative streamers are only weakly affected. Decreasing the pressure, in contrast, leads to longer propagation for both polarities and stronger streamer–streamer interaction. The simulations reproduce the qualitative pressure scaling, but they overestimate streamer velocities, predict earlier inception, and indicate that the modeled discharge extinguishes before the applied voltage pulse has fully decayed, suggesting that surface-charge accumulation, pulse-to-pulse memory, and three-dimensional effects are required for quantitative agreement.