I don't get much in depth information about the OC but here is the information I can offer courtesy of spoilerfix.com.
Episode 16 will air on March 9, 2006
Ryan and Sadie go on a road trip looking for her uncle Jack, who is Johnny's father and a bounty hunter. Marissa is involved in the investigation into Johnny's death. Summer does not yet know about her father and Julie.
Episode 17 will air on March 16, 2006
By this episode Ryan and Sadie and Marissa and Volchok are a couple. Seth is planning Ryan's birthday party. Dr. Roberts and Julie finally tell Summer about their relationship.