A related question: was an Operating System first known as a Time Sharing System? I've seen most of what I've known as an OS in this era termed that, also I read that the term OS was coined for CP/M.
IBM had OS/360 in 1966, about a decade before CP/M. Not sure where "OS" first appeared but it was before that.
Related: early OSes were not timesharing systems. They were batch oriented - you submit your stack of punch cards to the operator, and pick up your output later once they've had a chance to run it.
Operating systems abstracted the hardware (and other things) rather than having everything embedded in the program (as is typical in many embedded applications even still today). They don't have to offer time-sharing (aka, a scheduler), but that does constrain them to running a single process to completion before beginning the next. There were OSes before there was time-sharing OSes.
It would be useful to see if the software architecture of Whirlwind I [1] would have a piece that was like an OS. It is about a decade earlier than the IBM 360. Also it is a realtime system that tracks incoming airborne objects. This means that paradigms like "submit a batch job" are probably not an important part of using Whirlwind.
