Our good friends at tvguide.com have provided us with another guest-starring nugget. A new business/personal relationship looks to be forming for Kirsten:

Once and Again's Billy Campbell has been cast on The O.C. as Carter, a guy who comes to Newport Beach to work with Kirsten on a new, um, "business venture." (So that's what they're calling it now.) He'll appear in several episodes beginning March 3.