I had a look at your changes to OISPrereq.h, it looks fine, except, that the defines should come after the included header files. As, the only header files that are inclulded there are STL and OISConfig.h (which has no classes for exporting). ALso, OISConfig.h has the line that can be uncommented when using this as a DLL (OIS_DYNAMIC_LIB), so it needs to be included first.
Besides that, the changes are ok - though, you only had to declare "#define _OISExport" once. Because in the Win32 ifdef part, if it was defined (as blank) then it is undefined, and redefined as dllexport or import... Not really wrong, but doubles the line count unnecessarily.
edit - And I fine with the project files for 2005 being seperate.. But, as far as really old, I am using .Net 2002 (only a few years out of date) - and the same version of solution files that Ogre uses.. As it is compatiable with 2002, 2003, and 2005 (and CodeBlocks).. Not sure what problems you were having?