Both Canon 1D X II and Olympus E-M1 II have 20.0 MP resolution sensors but Canon 1D X II's sensor is Full frame (36 x 24 mm ) and Olympus E-M1 II's sensor is Four Thirds (17.4 x 13 mm ).
Since Canon 1D X II'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.
Below you can see the 1D X MII and E-M1 II sensor size comparison.
As seen above, Canon 1D X II has a 3.8x 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.