Olympus E-PL9 has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic VIII processor. On the other hand, Sony A9 II has a 24.0MP Full frame (35.6 x 23.8 mm ) sized BSI-CMOS sensor and features BIONZ X processor.
Sony A9 II's sensor provides 8MP more than Olympus E-PL9'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.
One other difference between these two cameras that is worth mentioning is that Sony A9 II'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 moire occurring in certain scenes.
Below you can see the E-PL9 and A9 II sensor size comparison.
As seen above, Sony A9 II has a 3.8x Larger sensor area than Olympus E-PL9. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.