Don't know if the commands are different. I know many of the core commands are not becuase I work on Mac's.
What do you mean 'core' commands? Bash is bash, csh is csh, etc etc.
Major differences are under the hood, package management, and some small things like start up scripts.
If you learn how to use one *nix in and out you'll be at home pretty much anywhere. Not to mention, each Linux distro has its own way of doing things.
And OS X is not the same as BSD, even though it borrows a lot.