Previous IBM 360 operating systems such as OS/MFT and OS/MVT had been written entirely in assembly language. The first IBM OS that had any significant portion written in PL/S was MVS, followed by OS/VS1, OS/VS2 and OS/SVS.
PL/S was part of IBM's OCO (http://www.landley.net/history/mirror/ibm/oco.html) (object code only) effort, started in 1983. PL/S was used internally and never released to the public. It is documented in various IBM internal ZZ-? publications.
Versions: PLS1, PLSII.
["PL/S, Programming Language/Systems", W.R. Brittenham, Proc GUIDE Intl, GUIDE 34, May 14, 1972, pp. 540-556].