The first operating system was developed in the early 1950s. It was known as GM-NAA I/O, created for the IBM 701 computer by General Motors’ Research division and North American Aviation. However, this was a rather rudimentary system by today's standards.
One of the first recognizably modern operating systems was IBM's OS/360, developed in the mid-1960s. It introduced the concept of a single operating system that could handle multiple tasks at once, which was a novel idea at the time. OS/360 was designed to provide a comprehensive software package that could run on all of IBM's then-current and upcoming mainframe models, albeit with some variance in functionality.
In between these two, various systems were developed to manage tape storage and job sequencing for batch processing systems, but whether these qualify as "operating systems" might depend on one's definition. Generally speaking, an operating system manages hardware resources and provides an environment for programs to run, and by that standard, systems such as GM-NAA I/O and OS/360 could be considered some of the first operating systems.