The Computers: Software: Operating Systems: (Closed Source Operating Systems )

This section lists operating systems (OSs) based on closed source code. Some such OSs have open source variants (BeOS, DOS, Unix, VMS), or straddle these two licensing models (QNX). Closed Source computer programs, including OSs, are those that include no source code, and for which the source code is kept secret, or made available only for a fee, as in many realtime OSs. Closed Source Operating Systems Software Computers.

Until the late 1960s computers—large and expensive mainframe computers, machines in specially air-conditioned computer rooms—were leased to customers rather than sold. (wikipedia)