To achieve sensitive detection of partial discharge in transformers and avoid missing partial discharge signals, a fiber optic omnidirectional sensing method for partial discharge in transformer oil combined with FBG and FP cavity is proposed based on fiber Bragg grating (FBG) and Fabry-Perot cavity sensing principle. The partial discharge sensing principle of FBG and FP cavity is introduced. The ultrasonic waves generated by partial discharge will cause a shift in the FBG reflection spectrum and the FP cavity interference spectrum, and the detection of partial discharge can be achieved by demodulating the optical intensity through the spectral edge demodulation method. A partial discharge fiber optic omnidirectional sensor was developed. A rectangular probe with the size of 25 mm×25 mm×25 mm was fabricated by 3D printing technology. The hollow structure of the probe was used to insert a single-mode fiber to form an FP cavity. In addition, the four sides of the probe were used to form a diaphragm-type FBG sensing structure, which can receive ultrasonic signals in different directions. The acoustic sensing diaphragm was designed based on the frequency spectrum characteristics of partial discharge in transformer oil and the vibration model of the diaphragm in the liquid phase environment. Corning glass coated with high reflectivity dielectric film was selected as the FP cavity sensing diaphragm with a radius of 1.7 mm and a thickness of 0.165 mm, and its theoretical resonant frequency is 82 kHz. Monocrystalline silicon was selected as the FBG sensing diaphragm with the radius of 2.5 mm and the thickness of 0.1 mm, and its theoretical resonant frequency is 25.6 kHz. The fiber optic sensing system for partial discharge was built, and the performance test was conducted on the partial discharge fiber optic omnidirectional sensor. The resonant frequency of FBG sensing diaphragm in transformer oil is about 23 kHz, and that of FP cavity sensing diaphragm is 71.4 kHz as measured by the pencil-break experiment. The fiber optic omnidirectional sensor was compared with the piezoelectric transducer (PZT) to detect the same partial discharge signal. 83.8 pC of metal tip discharge can be detected by the FBG sensing part and 27.1 pC of metal tip discharge can be detected by the FP cavity sensing part. The partial discharge detection sensitivity of fiber optic omnidirectional sensors is higher than that of PZT. The directional response of the sensor was tested. Ordinary FP sensors have a limited range of highly sensitive detection and a detection blind area. The developed partial discharge fiber optic omnidirectional sensor has the FP cavity sensing and FBG sensing parts. The highly sensitive detection range of the FP cavity sensing part is complementary to that of the FBG sensing part. The sensor can achieve omnidirectional sensitive partial discharge detection and has good directional response performance.