FreeBSD and OpenCL are the best of both worlds. FreeBSD is a light, capable, stable operating system. OpenCL opens up the world of parallel processing and ultra fast number crunching. Believe it or not the two can coexist and you can run OpenCL capable devices on FreeBSD, but it is a little different set up than the same with Linux where everything comes pre packaged.
To start with, I will assume you have a recently installed FreeBSD system that is up and running and that you know basic unix commands and package management tools. But first, a little explanation about how OpenCL works.