Panasonic G9 has a
20.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor . On the other hand, Olympus E-330 has a
7.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor .
Panasonic G9's sensor provides 13MP more than Olympus E-330's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
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 moiré occurring in certain scenes.
Below you can see the G9 and E-330 sensor size comparison.
Panasonic G9 and Olympus E-330 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-330 has 173% larger pixel area (30.47µm2 vs 11.16µm2) compared to Panasonic G9, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.