Mac OS 9 was written in Pascal, and so was the Berkeley P-System, a portable Pascal development environment from the 1970s, featuring a virtual machine that influenced the later JVM. Apple's Lisa OS was also implemented in Pascal.

Indeed, e.g. Apple Lisa OS and the first Mac OS. But the present one so far is only around 100 lines; the author calls it a "bootloader". Here is a list of more complete systems: https://wiki.freepascal.org/Operating_Systems_written_in_FPC.

My employer (France Telecom) had a Unix written in Pascal running on a 68000 machine. But there were all the usual Unix tools on it, including a C compiler.

I used Apollo Aegis (later Domain), Three Rivers PERQ and CDC NOS, all written in some variant of Pascal.