Well, adding to a specific OU is easy, but you say "where the user is".
Which user are you talking about? The notional one who might be using the
machine most of the time? The one that hasn't ever logged in to the machine
because it's not domain-joined yet? Automating something that you don't yet
know is always difficult.
Anyway, assuming you want to place a machine in a specific OU, here's how to
do it.
You need to find the netdom.exe utility which is located in the support
tools folder of your Windows CD.
I was going to work out an example, but it looks like somebody's already
done it for us.
http://www.tburke.net/info/suptools/topics/netdom_examples.htm
You will need to add parameters to the examples there to specify an account
and password. Just in case there's any doubt you should *not* use a domain
admin account here. Create a new account specifically for this purpose and
also a security group. Add the account to the security group and use the
Delegation of Control Wizard to delegate the right to add (and depending on
your needs, delete) computer accounts from the specific OU where you want to
place the machines.
Hope this helps
Oli