Both Olympus E-M1 II and Samsung NX300M have 20.0 MP resolution sensors but Olympus E-M1 II's sensor is Four Thirds (17.4 x 13 mm ) and Samsung NX300M's sensor is APS-C (23.5 x 15.7 mm ).
Since Samsung NX300M'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-M1 II.
Another difference between these two cameras is that Olympus E-M1 II's sensor lacks 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-M1 II and NX300M sensor size comparison.
As seen above, Samsung NX300M has a 1.6x Larger sensor area than Olympus E-M1 II. 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.