Doxygen can be download from:
GrpahViz's dot can be download from:
To build the Doxygen docs you'll need to set the environmental variable OSGHOME to the root of the OpenSceneGraph and then run doxygen with one of the Doxygen files supplied with the OpenSceneGraph distribution:
cd /home/myaccount/[=OpenSceneGraph export OSGHOME=`pwd` doxygen doc/Doxyfiles/core_Doxyfile
Then wait for Doxygen to build the documentation. Once built it will be placed in doc/OpenSceneGraphReferenceDocs from here you can browse the documentation.