1. Sequence of Events At the time of the incident, Plan X was in effect which means that aircraft are arriving runways 4R, 9R, and 9L and departing from runways 4L, 9L, 32R, and 32L intersection T10. The North Local controller was responsible for aircraft landing and departing runway 9L and departing from runways 4L and 32R. There were 3 controllers at the NLC position: north local controller (NLC), local monitor (LMN) and controller-in-charge (CIC). The LMN was responsible for monitoring the NLC frequency and watching runway intersections. Beginning approximately 90 seconds before UAL604 was instructed to hold on the runway until the incursion, the NLC was responsible for 11 aircraft. At 1507:15, the NLC instructed UAL604 to "position and hold [runway] 4L". The pilot acknowledged. At 1507:22, the NLC provided a departure sequence to American Airlines (AAL) flight 1425 that was acknowledged by the pilot. At 1507:31, the NLC issued departure instructions and cleared AAL1048 for takeoff on runway 9L that the pilot acknowledged. Eight seconds later, the NLC instructed Northwest Airlines (NWA) flight 117 to contact departure control; the pilot acknowledged. At 1507:44, the NLC instructed UAL 1520 to taxi across runway 4L and hold short of taxiway J, the pilot acknowledged. At 1507:59, the NLC cleared UAL604 for takeoff on runway 4L and instructed the pilot to turn right heading 090 after departure. The pilot acknowledged. Eight seconds later, the NLC instructed NWA1240 to "position and hold runway 4L" and the pilot acknowledged. The NLC then provided a departure sequence to American Eagle flight 51 (EGF51). At 1508:18, the pilot of UAL604 advised the NLC that they were aborting takeoff runway 4L. The controller then cancelled the takeoff clearance and instructed the flight to hold in position. The NLC stated in his interview that he typically turned over the flight progress strip of the departing aircraft when that aircraft is not able to depart as required by facility management. However, in this incident, he did not. He stated that he was sequencing other flight progress strips at that time and "forgot" to turn it over. The LMN was not monitoring the frequency as required. 2. Weather Information ATIS "Kilo" was broadcast as follows: O'Hare International Airport information Kilo, 1456 zulu wind 070 at 4 knots, visibility 10 miles, ceiling 2,600 broken, temperature four, dew point minus three, altimeter 30.25. Arrivals expect vectors ILS runway 4R approach, ILS runway 9L approach, ILS runway 9R approach. Parallel approaches in use. Read back all runway hold short instructions. Departures expect runway 4L, 9L, 32L, 32R, and 32L from taxiway T10 8,800 feet available. Notice to Airman: Runway 18/36 closed, lighted crane 155 feet above ground level 1.5 miles east of runway. Pilots use cautious for bird activity in the vicinity of the airport. When ready to taxi contact ground metering on frequency 121.67. Advise on initial contact you have information Kilo.