OM System OM-5 II has a
20.0MP Four Thirds (17.4 x 13 mm ) sized CMOS sensor and features TruePic IX processor. On the other hand, Olympus E-620 has a
12.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic III+ processor.
OM System OM-5 II's sensor provides 8MP more than Olympus E-620'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 OM-5 Mark II and E-620 sensor size comparison.
Sensor Size and Resolution Comparison image of OM System OM-5 II and Olympus E-620 Cameras
OM System OM-5 II and Olympus E-620 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.
Sensor size sets the circle of confusion, so it drives how much depth of field each
of these bodies gives you at a given aperture. You can calculate it exactly for the
OM System OM-5 II
or the
Olympus E-620 ,
including hyperfocal distance.