That depends a little on what you call a microcomputer and an operating system.
But typically, the first real microcomputer OS is considered to be CP/M (8-bit 8080). https://en.wikipedia.org/wiki/CP/M It was the first to interface a microcomputer with a floppy drive, and it was the most popular until it was dethroned by the IBM PC and DOS.
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But typically, the first real microcomputer OS is considered to be CP/M (8-bit 8080). https://en.wikipedia.org/wiki/CP/M It was the first to interface a microcomputer with a floppy drive, and it was the most popular until it was dethroned by the IBM PC and DOS.