The OG is the twelfth episode of the second season. It was released on 6 January 2006.
Charlie uses math to determine the killer of an undercover FBI agent, and explores the endless interrelation of the killings so often simply written off as 'gang violence'.
Tamara Taylor, who plays community center director Olivia Rawlings in this episode, went on to play Dr. Camille Saroyan in Bones; in that show she is head of the Forensic Division of the Jeffersonian and often works closely with the FBI to solve crimes. Aldis Hodge, who plays suspected shot caller Travis, went on to play Alec Hardison in Leverage; in that show he is a thief/hacker who, along with a team that includes a former FBI agent, works outside the law (usually) to solve crimes and right wrongs.
After Charlie explains to Don why he should look at the entire first shooting chain Don says you'd be safe shooting "Tim" to start a chain that leads to the killing of "Bob" because you have no motive for killing "Tim". However you DO have a motive for killing "Tim", it's just that your motive is less visible.
[This appears on the beginning of the episode] 1350 L.A. Street Gangs 152,000 Gang members 504 Homicides 2.2 square miles