GoPro Hero9 Black has a
23.0MP (1/2. ) sized CMOS sensor and features GP1 processor. On the other hand, Olympus TG-870 has a
16.0MP (1/2. ) sized BSI-CMOS sensor and features TruePic VII processor.
GoPro Hero9 Black's sensor provides 7MP more than Olympus TG-870'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 Hero9 Black and TG-870 sensor size comparison.
GoPro Hero9 Black and Olympus TG-870 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.