Apertos - Object-oriented reflective OS, from Sony Computer Science Lab.
Athene - Next generation, commercial, kernel independent, all object and component based computing environment and OS that users can design to their specifications. Backend is built to run in almost any environment.
Atomatrix - Multithreaded execution system of atomic matrices with dynamic compiler, a network environment, and an object-oriented OS. Open source, BSD.
Choices Object-Oriented Operating System - Research OS coded in C++, architecture organized as frameworks of objects hierarchically classed by function, performance; customized by replacing subframeworks, objects; application interface is kernel objects exported via application/kernel protection layer; kernel and application objects examined via application browsers; runs on bare hardware on distributed and parallel computers. Virtual Choices (VChoices) also runs under Unix System V.
Iguana Project - Investigating use of object-orientation, computational reflection, and metaobject protocols (MOPs) to support dynamic customizing of (system) software.
Maruti - Goal: create an environment for developing and deploying critical applications with hard real-time constraints in a reactive environment. Such applications must be able to execute on a platform of distributed and heterogeneous resources, and operate continuously in the presence of faults.
MK++ - Mach compatible microkernel; gives one code base able to support high assurance, scalability, realtime, distribution, SMP, fault handling, performance; all built in an object-oriented B3 evaluable fashion.
Renaissance - Multiprocessor OO OS purely designed and implemented in object-oriented techniques, to give application programs transparent access to system and remote objects distributed in a network of machines. OO programming is an ideal approach for building distributed systems. Runs on Sun SPARC, Encore Multimax multiprocessor.
ShagOS - Portable object-oriented microkernel OS, dynamically loaded device drivers, fully redesigned and rewritten many times in C++, runs on VAX, x86. Ongoing experiment in using O-O paradigm as framework for full OS, with distributed computing as main aspect in most design decisions.
SOS: SOMIW Object-Oriented Operating System - Most interesting research results: simple, generic, powerful object model; the concept of Fragmented Objects to structure distributed abstractions; developed flexible naming service, dynamic linking package, library of application oriented communication protocols.
Tornado - Object-oriented operating system for large-scale shared-memory multiprocessors. Minimizes data sharing via careful design: often eliminates data sharing even when sharing appears natural. Successor to Hurricane.
XOS - Distributed operating system, to be fully object-oriented, preemptively multithreaded/tasked, event/message-driven. Little code yet, but design documents and links are interesting, useful reading.