There are at least two interfaces to OpenGL for MLton/SML, both of which should be considered alpha quality.
MikeThomas built a low-level interface, directly translating many of the functions, covering GL, GLU, and GLUT. This is available in the MLton Sources: opengl. The code contains a number of small, standard OpenGL examples translated to SML.
ChrisClearwater has written at least an interface to GL, and possibly more. See
Contact us for more information or an update on the status of these projects.