Olympus E-PM2 has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor . On the other hand, Panasonic G9 has a 20.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor .
Another difference between these two cameras is that Panasonic G9'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-PM2 and G9 sensor size comparison.
Olympus E-PM2 and Panasonic G9 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-PM2 has 26% larger pixel area compared to Panasonic G9, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.