Olympus E-M5 III has a 20.0MP Four Thirds (17.4 x 13 mm ) sized MOS sensor and features TruePic VIII processor. On the other hand, Fujifilm X-T2 has a 24.0MP APS-C (23.6 x 15.6 mm ) sized CMOS X-TRANS III sensor and features X-Processor Pro2 processor.
One other difference between these two cameras that is worth mentioning is that Fujifilm X-T2's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the E-M5 Mark III and X-T2 sensor size comparison.
As seen above, Fujifilm X-T2 has a 1.6x Larger sensor area than Olympus E-M5 III. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.