You apply a GPO to an OU by simply linking to the GPO you've created at the appropriate level/OU in the Group Policy Management Console.
However, this will not work for what you're trying to do. GPOs pertaining to Password policies can only be set at the domain level. In order to apply a policy to a subset of domain users then you need to use Fine-Grained password policies.
These can be applied at the group level, so you need to ensure all the users you wish to affect with this new policy are a member of the appropriate group.
To do this on a Windows 2012 domain, do the following from a DC or Windows 8 workstation with RSAT installed:
- From the Start Screen type DSAC.EXE to start the Directory Service Administrative Center.
- Navigate to the System\Password Settings Container
- Right Click and select New or use New under the Tasks menu.
- Choose Password Settings
It's fairly self-explanatory from there.
If you're running on Windows 2008 or 2008R2 then you can still use Fine-Grained password policies, but it's not quite as easy to manage. To be honest, my advice at this level would be to think about upgrading but there are some guides around that might prove useful here too.
If your DCs are running an older version of Windows (or you're running newer DCs but the domain functional level hasn't been upgraded) then you've got no choice about upgrading at that point.