I used OS/2 from 1993 to 2003 almost exclusively. It has the most
beautiful GUI on the planet, is object-oriented to a fault, and is the
target of all the claims Microsoft was making with regard to the
Object-orientedness of their new windows 95.
Wikipedia
https://en.wikipedia.org/wiki/Workplace_Shell mentions some
important attributes of a truely object-oriented gui.
Someone mentioned inheritance and polymorphism. These are two products
of true object oriented gui design. Applications inherit the ability
to manipulate and use whatever objects exist in the system. A word
processor is not limited to just text files, for example, or to only
the files the programmer originally set out for it. The system allows
the applications to grow in functionality as new object types are
developed/assembled by other applications or the user.
I gather, though I have not had the opportunity to play with it, that
the Next Gui was also extreme in its object-orientedness, though I
can't see that from MACOS (its inheritor), I understood that to be the
case?
At any rate, if you want a fantastic example of a object-oriented
graphical user interface, check out the Workplace Shell.
Jeff