Two models are common in embedded systems: "main loop" or RTOS. In the main loop model, once initialization is complete, the program loops indefinitely, servicing various functions. Since the developer is responsible for a larger portion of the code, it is easier to meet timing deadlines.

An RTOS provides a convenience to allow various threads to block and thus simplify programming. I would choose an RTOS where there are a greater number of independent tasks which need to be performed.

Both are alternatives to a conventional OS which makes hitting deadlines difficult or impossible.