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A fiber-optic interferometric sensor with intrinsic transducer along a length of the fiber is presented for vibration measurements of the magnetic core inside an oil-filled power transformer. The sensor is designed for high sensitivity measurements into the harsh environment of electromagnetic fields, wide temperature change, and in oil immersing. This sensor allows enough sensitivity for the application, for which vibrations amplitudes are due to submicron displacements at frequencies of 100 Hz and its harmonics, in addition, the accessibility to the sensing region is guaranteed by low size immune fiber-optic cables as well as optical phase-based transducer/carrier of the sensor output. Thus, it is ideal for the requirements of the remote operation. The transducer size is a few mm in length, and is compact and rugged. Our results show that the vibrations of the elements constituting a power transformer are directly measured, improving the indirect techniques actually available. Finally, the measurements inside a medium power transformer at site are obtained for its application in electric plants.