There is a rich tradition of operating systems written in
Pascal. The probably best-known example is
classical Mac OS, a very influential historic operating system, that was nearly entirely written in Pascal (not to be confused with macOS that is written in Objective C)
Other operating systems written in Pascal include
Lisa OS and the
Lillith system software
The operating system for the famous
Xerox Star was written in the
Mesa programming language, a modular language of the Pascal family
Today there are several attempts to write a Pascal-based operating system from scratch. The following, probably incomplete, list contains some approaches to use Free Pascal for creating an OS