Objective optical assessment of photoreceptor function may permit earlier diagnosis of retinal disease than current methods such as perimetry, electrophysiology, and clinical imaging. Recent work with adaptive optics (AO) flood imaging, conventional OCT and phase-sensitive full-field OCT have revealed apparent changes in photoreceptor outer segment (OS) length in response to visible stimuli. In this work, we describe an AO-OCT system designed to measure these stimulus-evoked OS length changes. The OCT subsystem consisted of a Fourier-domain mode-locked laser that acquires A-scans at 1.64MHz and an AO subsystem providing diffraction-limited imaging with a closed-loop correction rate of 20Hz. To our knowledge this is the highest-speed AO-OCT system developed to date. Visible stimuli were delivered using a LED-based Maxwellian view channel incorporated into the system. In a dark-adapted healthy subject, 1-deg square volumetric images were acquired at a rate of 32Hz. Images were acquired for 10s, with a 10ms bleaching stimulus flash with variable intensity. Strip-based registration was used to track individual cones in the volume series, and time series of the resulting depth-resolved complex signal were analyzed. Stimulus-evoked changes in the morphology of OS and RPE were observed in the M-scan amplitude. In the M-scan phase, the difference between the IS/OS and COST was shown to increase in response to the stimulus flash, and the magnitude of the phase change depended upon flash intensity. These results suggest that cone OS elongates in response to visible stimuli, and that the length change scales with stimulus intensity.