Frank Barrus' ShagOS is a (running) OO system based on a message passing kernel. The OS itself hasn't received work in a while, however, this site is still regularily updated (note: no more, it seems), and it features several very nice tools, such as SOLO (a very advanced bootloader, that includes debugging, environment vars and filesystem support, and reminds me of Sun computer PROMs), and DECO (a "dynamic" C++ to C++ compiler).