We report the discovery of a 1-mHz optical quasi-periodic oscillation (QPO) in the candidate ultra-compact low-mass X-ray binary 4U 0614+091. The ultra-low-frequency QPO has no X-ray counterpart in contemporaneous RXTE/PCA data and is likely a signature of structure in the accretion disc. The QPO can be reasonably fitted with a single sine wave but with a phase jump part way through the observation, indicating that it is not coherent. We also identify a 48-min modulation, approximately consistent with the suggested orbital period of O’Brien and Shahbaz et al. If this is indeed orbital, it supports an identification of 4U 0614+091 as an ultra-compact source.