Nikon L31 has a 16.0MP 1/2.3' (6.17 x 4.55 mm ) sized CMOS sensor . On the other hand, Olympus TG-610 has a 14.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features TruePic III+ processor.
Below you can see the L31 and TG-610 sensor size comparison.
Sensor Size and Resolution Comparison image of Nikon L31 and Olympus TG-610 Cameras
Nikon L31 and Olympus TG-610 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. On the other hand, since Olympus TG-610 has 15% larger pixel area (2.04µm2 vs 1.76µm2) compared to Nikon L31, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.