Like almost everything in Computer science there would be no clear definition of first OS ever. There used to be monitor programs and schedulers before they iteratively evolved into OS programs. Just like, programming languages, servers, 'cloud'... There seldom is a first ever something in our field.
OS/360 was not the first OS. Quite a number of operating systems came before it.
Examples of earlier operating systems include SHARE Operating System (SOS), first version in 1959, for the IBM 709 mainframe; which in turn was itself was based on an earlier operating system, GM-NAA I/O, first version in 1956.
A very notable pre-OS/360 operating system was MIT's CTSS operating system, the first timesharing OS, first version in 1961 for the IBM 7094. The first versions of OS/360, by contrast, notably lacked timesharing – that was initially provided by alternative IBM operating systems (TSS/360 was the official answer and CP/CMS, which later became VM/CMS, the principal unofficial one); OS/360 only gained timesharing support itself when TSO was released in 1971.
