We run into this problem as well when we were migrating from MDT to OSP. MDT likes to use the built in admin account; you can configure OSP to do that as well, but it becomes difficult. As far as I am aware, your options are:
1.) The method we use: Use a domain service account opposed to the built in admin (domain\OSPaccount). This method forces you to use Unattend.xml to do domain joins prior to OSP picking the task back up after the unattended Windows installation.
2.) Use a registry import to configure autologin to login to the updated Administrator credentials (.\NewUserName) prior to your first reboot (I can provide an example reg file if desired)
3.) Avoid the domain join until the end of your template (there's a way to do it via VBS, but this is likely to be your hairiest choice, and one I'd avoid if at all possible)