Olympus E-PM1 has a 12.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic VI processor. On the other hand, Panasonic G85 has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor .
Another difference between these two cameras is that Panasonic G85'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 moire occurring in certain scenes.
Below you can see the E-PM1 and G85 sensor size comparison.
Olympus E-PM1 and Panasonic G85 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Olympus E-PM1 has 29% larger pixel area compared to Panasonic G85, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.