Olympus SP-820UZ has a
14.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CMOS sensor . On the other hand, Nikon B500 has a
16.0MP 1/2.3 (6.17 x 4.55 mm ) sized BSI-CMOS sensor .
Below you can see the SP-820UZ and B500 sensor size comparison.
Olympus SP-820UZ and Nikon B500 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 SP-820UZ has 15% larger pixel area compared to Nikon B500, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.