Olympus VG-120 has a
14.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features TruePic III processor. On the other hand, Nikon A100 has a
20.0MP 1/2.3' (6.17 x 4.55 mm ) sized CCD sensor .
Nikon A100's sensor provides 6MP more than Olympus VG-120'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.
Below you can see the VG-120 and A100 sensor size comparison.
Olympus VG-120 and Nikon A100 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 VG-120 has 44% larger pixel area compared to Nikon A100, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.