Olympus TG-870 has a 16.0MP 1/2.3" (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features TruePic VII processor. On the other hand, Nikon P7100 has a 10.0MP 1/1.7" (7.44 x 5.58 mm ) sized CCD sensor and features Expeed C2 processor. Olympus TG-870's sensor provides 6MP more than Nikon P7100'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 TG-870 and P7100 sensor size comparison.
As seen above, Nikon P7100 has a 1.5x Larger sensor area than Olympus TG-870. 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.