It seems like the point is to indicate what the switch does during the course of the score motor rotation (stay open, stay closed, pulse once on certain angles, pulse multiple times, etc). The first example on the image linked above seems incorrect - it should be ‘C’ to indicate it’s closed during the rotation.

That being said, I have no idea why the schematic snippet you posted shows a NO motor switch with ‘O’ by it. If that’s not just a single typo instance, then maybe that’s just how they labeled them instead of using ‘C’ (even though it seems completely counterintuitive).

Now I gotta go check my Space Mission schematics to see if they have this too ..