Raspbian *is* a "true OS" that runs on the Raspberry Pi. No OS compiled to run on x86 hardware is going to run on a Pi. If you want to run bsd unix, you will need to either find a package that someone has compiled and put together (a "distribution") or do it yourself. In any case, bsd unix is no more of a real OS than Raspbian is. Both are POSIX compliant.