Neuronal oscillations are a prominent feature of motor cortical local field potentials (LFPs) and abnormalities in oscillatory activity have been linked to several disorders. Theta-modulated gamma-frequency pulses of alternating current stimulation modulate motor learning in humans, however, it is unclear how these frequencies modulate motor cortical activity at the cellular microcircuit level. Here, we aimed to develop a method of bidirectionally modulating theta and gamma coupled oscillations in the motor cortex using closed-loop optogenetic stimulation of excitatory RBP4-Cre (retinol-binding-protein-4) or PV-Cre (parvalbumin) inhibitory neurons. Motor cortical LFPs were recorded in mice in which these neurons were transfected with Channelrhodopsin-2. Using our recently developed phase-tracking system (Oscilltrack), blue-light pulses were delivered at four phases of the ongoing motor cortical theta oscillation in awake, head-fixed mice. Optogenetic stimulation was delivered over a quarter of the theta cycle, either as a continuous pulse or a burst of three pulses at gamma frequency. Both stimulation types modulated theta power in a phase-dependent manner, with continuous stimulation demonstrating stronger modulation. Phase-dependent amplification during stimulation of excitatory vs inhibitory neurons was offset by 90°, in line with predictions from computational models of excitatory/inhibitory circuits. These findings demonstrate that phase-dependent modulation of theta power can be mediated by stimulation of excitatory or inhibitory neurons, and that the effect of specific stimulation phases is likely to be the result of interactions between these populations. This approach can be used to inform the development of brain stimulation methods to modulate these activities in humans.