Looks like a huge effort you've put into this project! I can see md3 and dds loaders too..
I tried both OpenTK 0.9.1 and 0.9.4, but got different error messages from both. For example, using 091 I got:
Error 2 'OpenTK.Graphics.TextureFont' does not contain a definition for 'MeasureText' and no extension method 'MeasureText' accepting a first argument of type 'OpenTK.Graphics.TextureFont' could be found (are you missing a using directive or an assembly reference?) C:\Documents and Settings\Administrator\Desktop\oogl\oogl\trunk\OOGL\GUI\Abstract\BaseFrame.cs 463 40 OOGL.GUI
[ One way to simplify this kind of thing, is to include OpenTK in the distribution of OOGL on sourceforge.net. I don't think it violates any license.. ]