@joshua_mchugh George’s mention of using a post install script to do it is more advanced but very worth the effort. Having it domain joined via sysprep specialize simplifies things in the long run.
That being said, you’re probably misunderstooding groups, because they’re a little confusing. Groups in Fog do not dynamically update the OU of the host members, but it can be used to set the OU in bulk on members. There is a plugin to change the behavior of groups if you want, but I’d try it the normal way first.
But if you set the OU on the host, then when it joins the domain via the fog client, it will be in that OU. It will not move a host to a different OU, unless you do something like manually leave the domain and change the computer name and then the fog service will rename the computer back to what it is in fog and then join the domain in the set OU.
I personally use a post install script now that grabs the OU from to host and Injects that into my unattend file. I believe I’ve posted some examples in the past. If I remember tomorrow when I’m at a computer and not a phone, I’ll link them.