Nikon L31 has a 16.0MP 1/2.3' (6.17 x 4.55 mm ) sized CMOS sensor . On the other hand, Olympus 550WP has a 10.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor .
Nikon L31's sensor provides 6MP more than Olympus 550WP'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 L31 and 550WP sensor size comparison.
Sensor Size and Resolution Comparison image of Nikon L31 and Olympus 550WP Cameras
Nikon L31 and Olympus 550WP have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.