 Grey , Yang , Karev , and Stevens are all promoted to residents , in the season four premiere , while O 'Malley is forced to repeat his internship year , following his failing of the intern exam . Subsequently , Torres and O 'Malley divorce one another , due to him having a sexual affair with Stevens , initially concealing it from Torres . Early in the fourth season , cardiothoracic surgeon Erica Hahn becomes Torres ' love interest . During the fifth season , Hahn departs from the series , and O 'Malley retakes his intern exam , passing , joining his fellow physicians as a resident . Two new characters are introduced , former United States Army trauma surgeon Dr. Owen Hunt , and pediatric surgeon Dr. Arizona Robbins . Hunt becomes a love @-@ interest for Yang , while Robbins becomes a love @-@ interest for Torres . When Stevens is diagnosed with stage four metastatic melanoma , she and Karev wed one another at the conclusion of the fifth season . In addition , Meredith and Shepherd marry , with their vows written on a post @-@ it note . 
