On September 11, 2001, O'Brien was flying a Minnesota Air National Guard C-130H (Hercules) cargo airplane using callsign GOFER06.

Washington Reagan National Airport air traffic control asked O'Brien to identify the aircraft.

Controllers asked ("vectored") O'Brien to follow the plane (later identified as the errant AA Flight 77) as it approached Washington, D.C., from the west.

He reported to the control tower, "Looks like that aircraft crashed into the Pentagon, sir,"[1] and afterward remarked that it had hit the west side.

When crossing western Pennsylvania at about 10:00 am, local air traffic control asked them to try to spot another errant aircraft, Flight 93.