To answer the question directly: George O'Malley dies in the season 5 finale, "In the Air Tonight," which aired on May 14, 2009. The episode marks a heart-wrenching moment in the series, as George sacrifices his life to save a woman.
He is brought into the hospital as a "John Doe" because his injuries left him completely unrecognizable. The staff at Seattle Grace, including his closest friends, treat him for hours without realizing who he is. when does george die in greys anatomy