Olympus E-M1X has a 20.0MP Four Thirds (17.4 x 13 mm ) sized CMOS sensor and features Dual TruePic VIII processor. On the other hand, Panasonic S1 has a 24.0MP Full frame (35.6 x 23.8 mm ) sized CMOS sensor and features Venus Engine processor.
Another difference between these two cameras is that Panasonic S1'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 moire occurring in certain scenes.
Below you can see the E-M1X and S1 sensor size comparison.
As seen above, Panasonic S1 has a 3.7x Larger sensor area than Olympus E-M1X. 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.