Olympus E-620 has a 12.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic III+ processor. On the other hand, Panasonic G85 has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor .
One other difference between these two cameras that is worth mentioning is that Panasonic G85'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-620 and G85 sensor size comparison.
Olympus E-620 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-620 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.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Olympus E-620 and Panasonic G85 sensors have been tested by DxO and the results show that G85 has a better overall score of 71, 16 points higher compared to E-620's score of 55.