Set a new GPO on that OU to set the screensaver for 120 minutes. The OU screensaver policy will take precedence.
Password policies in 2003/2000 are a special case. Only one password policy per domain and it has to be linked at the domain level so that will always win. An OU level password policy will not have any affect on domain accounts. So you will be ok in terms of the password and account policies.
Thanks
Mike