Sony A9 has a 24.0MP Full frame (35.6 x 23.8 mm ) sized Stacked BSI-CMOS sensor and features BIONZ X processor. On the other hand, OM System OM-1 has a 20.0MP Four Thirds (17.4 x 13 mm ) sized Stacked BSI Live MOS Sensor sensor .
One other difference between these two cameras that is worth mentioning is that OM System OM-1'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 moire occurring in certain scenes.
Below you can see the A9 and OM-1 sensor size comparison.
As seen above, Sony A9 has a 3.7x Larger sensor area than OM System OM-1. 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.