Olympus E-P7 has a
20.0MP Four Thirds (17.4 x 13 mm ) sized CMOS sensor and features TruePic VIII processor. On the other hand, Nikon D750 has a
24.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Expeed 4 processor.
Another difference between these two cameras is that Olympus E-P7's sensor lacks 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 moiré occurring in certain scenes.
Below you can see the E-P7 and D750 sensor size comparison.
As seen above, Nikon D750 has a 3.8x Larger sensor area than Olympus E-P7. 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.