Multics & PL/I
Multics was written in (a version of) PL/I with the added challenge of having to develop the compiler in parallel with the operating system.
IBM has written chunks of its various operating system using PL/S and its successors.
VAXELN, a bit of a curiosity in the Digital Equipment Corporation range of operating systems, was written in Pascal. Chunks of VMS were written in BLISS-32 (though a lot of it was Macro-32).
Biting the hand that feeds IT
