I have heard Rob Pike describe Plan 9, or at least the kernel, as "The most object-oriented system ever built," (apologies if I paraphrased incorrectly). He was referring to the fact that everything satisfies one simple interface, and so everything can be plugged into almost anything else.
Right, everything is a filesystem in Plan 9. It's really elegant. Many people think Unix is elegant, which it is, but then they realize there is a higher plane of elegance in interface design when they encounter Plan 9.
In case anyone is wondering how the Plan 9 approach could accommodate special devices and so forth (i.e. objects that do not conform exactly to the same interface), there is an escape hatch called a control file, and you can do the equivalent of an ioctl by writing device-specific commands to the control file. But by and large, P9 represents everything as a filesystem and thus makes it really easy to reconnect the piping any which way the user likes.
BeOS and Plan 9 have such different concepts of OO that they're hard to compare. In BeOS, there's a C++ interface your applications have to conform to, and it's somewhat machine-centric. Your objects are live processes in memory.
Plan 9 guys tend not to like the design of HTTP ( http://http02.cat-v.org/ ) so don't tell anyone I used this illustration, but Plan 9 is OO similar to the way REST is.
In Plan 9 (and Inferno) your objects can be files on disk, structs in memory, devices, etc. Anything that can conform to the interface. The interface in this case is files. Your objects expose a filesystem, and you interact with them by means of the usual tools for file manipulation. The Acme editor exposes a filesystem, the clipboard ("snarf buffer") is a file, and the network is, instead of a group of special syscalls, a filesystem. So you could write a shell script that reads a URL from the snarf buffer, downloads the page it points to, and puts the content into a new window in Acme, in three or four lines, without using anything more complex than cat and echo.
