Before you can easily use ZFS pools in both OS X and FreeBSD, you have to make sure they belong to the same user and group.
So, if you install ZFS on OS X and you put FreeBSD after that, it’s quite easy, just create the same username as you have on OS X and add him to the staff group. After that, while still in FreeBSD (it’s easy from here), you’ll have to change your user ID to 501 (or whatever you have in OS X, but 501 is the default one). If you don’t know your user ID (UID), just run
% ls -aln
in your home folder folder.
If you had FreeBSD already installed, you’d probably have UID 1001. Let’s change that to 501.
% sudo pw <username> 1001 -u 501
Now add your user to staff group, but be sure not to stay in wheel group:
% sudo pw usermod <username> -G staff,wheel
Almost set. Just take ownership of the file from the pool
% sudo chown -R <username>:staff /STORAGE
My pool is called STORAGE and it’s mount on /. Your details may differ, but you got the idea. That’s pretty much it. Now you can enjoy read an write access from both OS X and FreeBSD, as God indented to.