Droplets, particularly water droplets, are abundant in both natural and artificial systems. Their capillary oscillations are governed by surface tension and are therefore distinguished from acoustic oscillations. These capillary oscillations play a major role in droplet coalescence, for example, and are also an important phenomenon in interface theories. Here, we experimentally and theoretically analyze the capillary oscillation within an optical cavity with walls of water. Our droplet benefits from an optical finesse of 520 that, accordingly, boosts its sensitivity in recording Brownian capillaries with amplitudes of 1 0.025 Å and kilohertz rates in agreement with naturalfrequency calculations. Our hybrid device allows resonantly enhanced interactions between electromagnetic and capillary waves that could potentially lead to optical excitation or the cooling of droplet capillary oscillations. © 2016 Optical