OpenGL, a high performance framework for computer graphics, is an important skill for anyone serious about developing for virtual reality. While Unity and others provide a very friendly development platform, the next level of developing truly special products is much more accessible for someone fluent in OpenGL.
There is a great resource at this site. Learn OpenGL houses a lot of tutorials and resources for those wanting to get up to speed quickly. Experience with C++ is helpful, but basic concepts can be attained without fluency in the language.