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We report a novel type of fully integrated optomechanical sensor and demonstrate high sensitivity mechanical displacement measurements on chip. We sense the motion of micro- and nano-mechanical devices by near field coupling them to high quality factor optical microdisk resonators. In a first geometry, we sense the position of a dielectric ring moved by a micromechanical actuator. Tunable optomechanical coupling of up to GHz/nm results in the photodetector-limited displacement sensitivity of 4.3×10-15 m//√Hz with optical excitation power as low as 20 μW. In a second geometry, we sense the thermal motion of a nanoscale cantilever probe. Transduction of the cantilever's MHz frequency vibration is achieved with a displacement sensitivity of 4.4×10-16 m//√Hz and a mechanical quality factor of 6000 is measured in vacuum.