The 2005 University of Oklahoma bombing occurred on October 1, 2005 at approximately 7:30 p.m. CDT, when a bomb went off near the George Lynn Cross Hall on Van Vleet Oval on the University of Oklahoma (OU) main campus. The blast took place less than 200 yards west of Oklahoma Memorial Stadium, where 84,501 spectators were attending a football game. The bomber, OU student Joel "Joe" Henry Hinrichs III, was killed in the explosion; no one else was killed.

After the incident, speculation began immediately about Hinrichs' motivation and the number of accomplices, if any. An off-duty police officer had noted Hinrichs talking with a local feed store owner about ammonium nitrate and had begun an investigation. There was innuendo regarding the Pakistani heritage of the bomber's roommate. Terrorism-related speculation was heightened after the explosive agent was found to be TATP, an explosive of similar manufacture, though separate makeup, to the compound used in the July 2005 London bombings, the attempted destruction of an airplane by a shoe bomber, and by Hamas in clandestine devices for use by suicide bombers. The Federal Bureau of Investigation (FBI) and local authorities concluded in 2006 that Hinrichs acted alone and had no assistance from other groups. They could not prove or disprove allegations that Hinrichs intended to enter the packed stadium and kill football fans along with himself.

Shortly before halftime of the Saturday evening game between the Oklahoma Sooners and the Kansas State Wildcats, spectators inside the stadium heard an explosion. The explosion was reported as being heard up to five miles away, but people on the east side of the stadium heard only a rumble like low-volume thunder. Some heard nothing. Spectators were not allowed to leave the stadium at half-time, which caused concern among fans who did hear the explosion. No stadium announcements were made until the third and fourth quarters of play, when bus drivers were asked to leave the stadium and meet at the northwest corner of the Asp Avenue Parking Facility. During the last six minutes of the game, an announcement was made over the public address system in the stadium and over local radio for attendees to exit through the south and east gates, and to avoid the area, known as the South Oval, west of the stadium. University officials had decided against giving any further explanations to game attendees because they "didn't want to start any kind of panic."

"Everyone in my section heard it [the explosion]. We looked to the opposite side of the field though thinking it was thunder. Me and my family were gonna go to the university bookstore after the game but the direction we had to go was blocked by caution tape. We knew something was up and got out of there ASAP." said a Sooner fan.

First reports said the explosive device was made using hydrogen peroxide, though as later noted this was only a component of the actual explosive compound used. The initial accounts also indicated a second bomb was found, but these accounts were false. The area was searched by bomb-sniffing dogs, and no more explosives were found. The remains of the backpack contained a circuit board, wires, and a battery; a small explosive was used to safely detonate it at about 9:00 p.m. Other items at the bomb site included a crescent wrench, a white sock with protruding wires, a screwdriver, unused wooden matches, and a chemistry book.

Following the bomb explosion, the entire South Oval was marked as off-limits with crime scene tape. Fans exited the stadium at the conclusion of the game without incident. The South Oval was open the next afternoon except for the immediate area of the explosion, so a broken glass door could be replaced and firefighters could finish spraying down the area with water to wash debris, chemicals and bodily fluids away from the sidewalk and a bus parked there before the preceding day's game. Classes resumed as normal on Monday.^{[citation needed]}

In a press conference the next day, OU president David Boren identified Hinrichs, a 21-year-old mechanical engineering student, as responsible for the detonation. Originally from Colorado Springs, Colorado, Hinrichs was a National Merit Scholar who graduated from Wasson High School in May 2002. His father, Joel Hinrichs Jr., described him as a "very private individual" who had gone through "several severe bouts of depression". Hinrichs' depression was noted as early as ten years old. His father believes that the "underlying cause was his inability to bond with other people. He couldn't make friendships," and also said that Hinrichs began counseling at the university's on-campus health center two years earlier. He did not know if his son was still seeing a counselor, and denied knowledge of the young man's suicidal ideations.

Hinrichs was a member of Triangle Fraternity, a social fraternity of engineering, science and architectural majors. Hinrichs' father said that his son had recently moved out of the Triangle fraternity house because "he didn't bond, didn't relate well to the other fraternity members." In the chapter meeting following the bombing the members of his fraternity were asked to direct all questions and comments to one member of their fraternity and not to discuss anything with the media; this was advised in hopes of limiting rumors and other theories.