Both Pentax X90 and Olympus 1 have 12.0 MP resolution sensors but Pentax X90's sensor is 1/2.3-inch (6.08 x 4.56 mm ) and Olympus 1's sensor is 1/1.7-inch (7.44 x 5.58 mm ).
Since Olympus 1'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 Pentax X90.
Below you can see the X90 and 1 sensor size comparison.
As seen above, Olympus 1 has a 1.5x Larger sensor area than Pentax X90. 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.