For industrial control applications, you typically use an existing real-time OS (anything from a small thing like FreeRTOS to something fancy like QNX) or you use a "PLC" which is a "black box" device that you can program with behaviors that it knows how to drive servos from. The PLC, in turn, either just uses embedded firmware, or in turn uses a real-time OS.
Perhaps you meant something else when you say "OS"?