After some minor code changes, I was able to load up the "O2 - All Active Projects" solution file inside OSx using MonoDevelop and Mono 1.2.6.
To replicate this should be just a case of doing a SVN checkout to http://o2platform.googlecode.com/svn/trunk/ (or http://o2platform.googlecode.com/svn/trunk/O2%20-%20All%20Active%20Projects/ directly) and compiling the "O2 - All Active Projects" solution file (you can also open up the individual "O2 Tool - xxx" solution files.
The command line modules work OK, but the problem seems to be on the WeifenLuo.WinFormsUI module (which is the one that recreates the VS-like windows docking environment), namely its PInvokes.
When running any one of the O2 Tool modules (either from MonoDevelop, or from the command line using "mono _FindingsViewer\ %28O2\ Tool%29.exe") I get the error: "Unhandled Exception: System.DllNotFoundException: user32.dll"
I google this a bit and there seems to be some hacks around this (one of them seems to involve using WINE).
Is there a way to solve this, or do I need to see how hard it will be to remove the PInvokes from the WeifenLuo.WinFormsUI dll or find another GUI host environment for O2?