Olympus E-M1 III has a
20.0MP Four Thirds (17.4 x 13 mm ) sized CMOS sensor and features TruePic IX processor. On the other hand, Nikon D500 has a
21.0MP APS-C (23.5 x 15.7 mm ) sized CMOS sensor and features Expeed 5 processor.
Another similarity between these two cameras is that both Nikon D500 and Olympus E-M1 III sensors lack anti-alias (Low-Pass) filters. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the E-M1 III and D500 sensor size comparison.
As seen above, Nikon D500 has a 1.6x Larger sensor area than Olympus E-M1 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.