Both Olympus E-M10 III and Pentax K-5 IIs have 16.0 MP resolution sensors but Olympus E-M10 III's sensor is Four Thirds (17.4 x 13 mm ) and Pentax K-5 IIs's sensor is APS-C (23.7 x 15.7 mm ).
Since Pentax K-5 IIs's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Olympus E-M10 III.
One other difference between these two cameras that is worth mentioning is that Pentax K-5 IIs's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the E-M10 III and K-5 IIs sensor size comparison.
As seen above, Pentax K-5 IIs has a 1.6x Larger sensor area than Olympus E-M10 III. 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.