I did a lot of searching around and experimenting, but I can't seem to get windowed mode to work for OFF.
I've found that CFS3 itself supports windowed mode by adding -window to a shortcut to cfs3.exe, although that doesn't work out to well for OFF since it leaves out the manager, which is critical. Adding -window to a shortcut to the manager doesn't work either.
The only program I've found which forces games to run in windowed mode is DXwnd, but I couldn't get it to work despite following the instructions to the letter.
I can't find any windowing options in the manager, configuration for resolution, graphics settings, override settings, or in the XML files themselves in the OFF directory.
I realize windowed mode isn't really an intended function of OFF, and OFF runs perfectly for me otherwise (in fact, it's a blast). However, it would be useful in the context of broadcasting my flights via uStream, such as I've done for Black Shark and OpenFalcon. The reason why windowed mode is important is that the output for most games flicker on the stream if they run in full-screen mode, but render properly in windowed mode (even if the window is stretched to fill the entire screen).
In any case, I just got OFF recently, and I wanted to state in this (as my first post) that it's been downright addicting. The open cockpits of these aircraft feel like a better environment for TrackIR than any other. I've managed to (barely) survive a couple campaign missions so far, and it's been good fun. :D