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 .
One other difference between these two cameras that is worth mentioning is that Panasonic G9'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 moiré occurring in certain scenes.
Below you can see the E-PM2 and G9 sensor size comparison.
Olympus E-PM2 and Panasonic G9 have the same 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.