> Mac OS was a mix of Object Pascal (originally created by Apple), Assembly and C++.

That doesn't ring true at all.

Object Pascal was a rather short-lived project at Apple. It was only seriously used for the MacApp framework -- which was a separate product sold to application developers, not part of the core OS or development tools -- and was abandoned entirely during the PowerPC transition. Later versions of MacApp used C++.

The bits of source code I've seen for System 7 were primarily C and assembly, with some older code in (non-object) Pascal. I don't recall seeing any C++.

I checked through the source code for System 7.1 at https://www.macintoshrepository.org/1398-mac-os-7-1-source-c... . You look correct (A=assembly, H=header, C=c, P=pascal, R=resource)

    % find ~/Downloads/System\ 7.1\ Source/ -type f -print0 | xargs -0 -n 1 basename | cut -d. -f 2- | tr '[a-z]' '[A-Z]' | sort | uniq -c | sort -rn | head -20
      869 A
      308 H
      189 C
      162 P
       85 R
       69 MAKE
       31 M.A
       28 O
       11 RSRC
       11 AII
    ...