We propose and demonstrate hydraulic pressure measurement using an optical frequency domain reflectometry technique in combination with a fiber-optic sensor for all optically real-time geophysical observatory system on deep seafloor. The hydraulic pressure causes a change in the optical strain of the fiber sensor. The sensor was constructed as Michelson interferometer with dispersion-shifted fiber with 2615-m in roundtrip optical path difference, an optical coupler and two faraday rotator mirrors. It was rolled on a hollow mandrel with an aluminum-alloy material to obtain a linear response as elastic characteristic under the high pressure of 4,000-m depth. The sensitivities of this sensor were -0.0234-ppb/Pa and 28.5-ppm/degree-C, respectively, in hydraulic pressure and water temperature. To detect this slight strain, we used a frequency-shifted feedback laser as an optical source. In our experimental setup, we obtained standard deviation of 9.6-ppb for optical strain measurement with two seconds period each; it corresponds to 41-mm in pressure measurement with above-mentioned sensor and a thermistor temperature sensor for temperature compensation. Further improvement of the measurement accuracy is possible based on this first preliminary experiment.