The only child of Jack and Sarah O'Neill, who was killed as a boy, having accidentally shot himself with his father's gun. Charlie's memory continues to plague Jack, who has told his team mates that he can never forgive himself for what happened.
Charlie loved spending time with his dad, and was very fond of outdoor activities, especially baseball. He loved to laugh. His abrupt death occurred moments after Jack had arrived home one day.
It is speculated that Charlie's middle name may be "Tyler," as an award certificate on his dresser in "Stargate" the Movie reads "Tyler O'Neil."
"Stargate" the Movie - Charlie's death paralyzes O'Neill until General West offers him an opportunity to end his misery by commanding the first Stargate mission. Jack intends to destroy the Abydos gate, and himself along with it.
Cold Lazarus - A member of a crystaline alien species takes the form of Charlie, giving Jack encouragement that his son still survives within his heart. (Kyle Graham)
The Devil You Know - Under the hallucination caused by the Blood of Sokar, Apophis uses O'Neill's memories of Charlie to try and gain information about the Ancient's memory download and the Asgard. (Dillon Moen)
Unnatural Selection - First, the leader of a highly advanced generation of Replicators, uses the painful memory of Charlie's death against Colonel O'Neill.