GEOS is described as the worlds first ObjectOriented OperatingSystem. Originally developed by GeoworksCorporation?, and now maintained by BreadboxComputerCompany.
It is apparently written using an ObjectOrientedAssembler designed by Geoworks for the purpose.
GEOS Faq, GEOS History
Interview w/ Adam de Boor, former CTO of GeoWorks?:
The first time I ever saw GEOS, it was running on and Apple II, and working pretty much like AppleMacintosh did, but on much more limited hardware, and in full color. I was very impressed that they managed to pull off this feat. Since then, GEOS has become available on other platforms, including 8086 PCs and portable devices.
GEOS was also used for a line of PDAs called the Zoomer.