Opie (Open Palmtop Integrated Environment) is the default graphical user interface that runs on top of OZ, and includes standard apps like a contact manager, calendar, clock and loads more. Opie was originally designed for handheld computers and is based on Qt/Embedded, which itself is a graphical environment designed for embedded applications based on the Qt toolkit from Trolltech.
You can read all about Opie at its home page, where you can find an overview of Opie, a user manual, TODO list, screenshots, and so on.
If Opie crashes, you can always restart it from the command line with:
# /etc/init.d/opie start