According to Wikipedia, the first operating system was GM-NAA I/O, produced in 1956 by General Motors' Research division for its IBM 704.
According to Merriam-Webster and Etymonline, the term "operating system" was first used in 1961. (Neither includes a citation)
The Oxford English Dictionary has the actual 1961 citation: 1961 Ann. Rev. Automatic Programming 2 232 Two operating systems are provided in the Honeywell package.
So there's about a five year gap before the terminology emerged after the technology arrived. Then again the term may have only been used sporadically at first and only settled into place some years later?
In any case, which operating system was the first to be called an operating system at the time? I realize there may be a difference between people in the field using the term and companies producing and/or marketing them to use the term.
As a bonus question, what other terms were used for the early OSes before the terminology did settle? I would guess maybe monitor and kernel might be among them?
It seems my wording was not clear. I am looking for the first use of the term and to which operating system that first use referred. I am not looking for later texts on the subject of early operating systems which existed prior to the publication of that text.
(If anyone can help me word this clarification more clearly, please do. Stephen Kitt has understood my intention perfectly.)