Dr. George O'Malley
Season 1
Dr. George O'Malley was one of Meredith Grey's roommates. He had an infatuation with Meredith from his first meeting with her, but she continually fails to notice, or chooses to ignore, his affection.
During the beginning of his internship, he was picked by Dr. Preston Burke to perform the first surgery of all the interns. During the surgery, he choked and earned the nickname "007" (after James Bond's License to Kill). In the next season, George and Alex Karev get stuck in an elevator with a patient who has been shot. The patient begins to bleed out and when Alex freezes up, Burke has to talk George through surgery and George "flies solo" for the first time.
George starts to date a nurse named Olivia in Season 1, from whom he contracts syphilis.
Season 2
George finally admits his feelings to Meredith, and they impulsively decide to sleep together. Halfway through, Meredith breaks down crying, leaving George mortified. Their friendship is strained momentarily, and after accidentally falling down the steps to avoid Meredith, he dislocates his shoulder. When he is getting it checked out, he meets and starts to date Dr. Callie Torres, an orthopedic surgeon at Seattle Grace.
Season 3
During a camping trip, he learned from Alex that Callie had slept with Dr. Mark Sloan, and also witnessed one of Burke's hand tremors. He confronted Cristina with his suspicions about Burke's problems after his father was diagnosed with esophageal cancer and a leaking aortic valve that would require surgery.
Concerned about Burke's current abilities as a surgeon, he contacted Dr. Erica Hahn to perform his father's valve replacement. His relationship with Callie was strained when he confronted her about her fling with Sloan, but as his father's condition deteriorated, George slowly allowed Callie to re-enter his life. George suffered a devastating loss when the difficult decision was made to take his father off life support after complications from cancer surgery left him in multi-system organ failure. Afterwards, Cristina confides to George that her father died when she was very young, and that he (George) is now a member of the "Dead Dads Club". She notes, somberly, "I'm sorry you had to join the club", before walking away.
In an attempt to overcome the grief he experienced from his father's death, he secluded himself with his girlfriend and pursued a highly sexual relationship. He asks her to marry him and they eloped to Las Vegas. Callie continually insists that George favors his friends (Izzie in particular) over her, and they had a serious fight where Callie told George that Izzie has feelings for him, and ultimately kicked him out of their hotel room. George then went to Izzie, confessing that he feels he may have made a mistake in marrying Callie, and ends up having sex with Izzie while the two are heavily inebriated.
George is torn over what to do about both Izzie and Callie. He put in for a transfer to nearby hospital Mercy-West, thinking the only way to resolve his feelings for Izzie is to be away from her. His request is granted, barring his success on the intern exam. After Izzie realizes how conflicted George is and not wanting to lose her best friend, she lies to George and says that sleeping with him meant nothing and that she doesn't have feelings for him. In the finale of the show's third season, Izzie admitted to being in love with George. It was revealed that George failed his intern exams.
Season 4
By the time the summer ended, George seemed to have changed his mind and went ahead with repeating his internship with Meredith as his resident. He also has a change of heart about his feelings for Izzie and tells her he loves her. Later, he finally tells Callie that he had an affair with Izzie. Callie tried to forgive him and shift all the blame to Izzie, but George says she cannot forgive what he did; Callie ends up agreeing. Callie then bluntly tells the rest of the hospital about George and Izzie. He breaks up with Izzie soon after due to a lack of chemistry, but continues to be good friends with her.
Initially, none of the interns knew that George was a repeat intern. Lexie kept his secret, and stayed by his side as a friend when all of the other interns find out about it. He moves in with her and starts accepting and living with what he has, befriending the other interns and hosting a party at their ugly apartment. In the season 4 finale, it is revealed that George failed the intern exam by only 1 point and received 14 outstanding letters of recommendation, making him and Lexie doubt his repeated internship even more. During a brief outburst at the chief, George is allowed to re-take the intern exam. When he comes home to Lexie, they share a brief celebratory kiss.
Season 5
George finally passes his intern exam, after a little difficulty from a hospital flooding. Also during this time, he seems to treat Lexie with even less respect, someone who has been harboring feelings for him since the season 4 finale. The final straw occurs when he receives his new interns and Lexie discovers he hadn't the decency to even ask to have her assigned to him. She breaks down in front of him, calling herself stupid and him, a jerk.
George tries to patch things up between them afterward, but Lexie starts to avoid him. Despite this, she finally decides to apologize and give a peace offering at the end of the night. Their friendship suffers nonetheless, in which she shuts him out of her life and doesn't tell him what's going on; even when she participates in a secret intern society. He's tried to refocus on his friendship with Izzie, afterward, when he begins to worry that something's truly wrong with her. George asked Izzie many times if there was something wrong with her and he was angry and disappointed that she chose to go to Cristina with her sickness and not himself.
Source: Wikipedia.org