Olympus E-M10 IV has a 20.0MP Four Thirds (17.4 x 13 mm ) sized CMOS sensor and features TruePic VIII processor. On the other hand, Nikon D7500 has a 21.0MP APS-C (23.5 x 15.7 mm ) sized CMOS sensor and features Expeed 5 processor.
One other difference between these two cameras that is worth mentioning is that Nikon D7500'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-M10 IV and D7500 sensor size comparison.
Sensor Size and Resolution Comparison image of Olympus E-M10 IV and Nikon D7500 Cameras
As seen above, Nikon D7500 has a 1.6x Larger sensor area than Olympus E-M10 IV. 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.