The role of theta oscillations in the orbitofrontal cortex during reward-guided behaviour is unknown. Here, Knudsen and Wallis used closed-loop control to record these oscillations to enable disruptive microstimulation of the oscillations in two macaques trained in a two-alternative forced-choice task in which reward contingencies were gradually changed over a number of trials. The phase alignment of theta oscillations between trials increased during value-updating trials, particularly during the fixation epoch. Disruption of theta during this epoch impaired the animals’ ability to update their choices, implicating these oscillations in value updating.