DURING TKOF AT ORD RWY 09R ANOTHER ACFT ON APCH TO RWY 04R COMMENCED A GAR. AT APPROX 120 KTS ORD TWR CANCELED TKOF CLRNC. CAPT REPLIED UNABLE. SHORTLY AFTER LIFT-OFF HE TURNED TO ASSIGNED 140 DEGS. SAW TFC 1 O'CLOCK HIGH; LOWERED NOSE AND PASSED BENEATH WITHOUT INCIDENT.

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DURING TKOF IN A 150000 LB ACFT AT ORD ON RWY 09R ANOTHER ACFT ON APCH TO RWY 04R EXECUTED A GAR. AT APPROX 120 KTS ORD TWR CANCELED OUR TKOF CLRNC; CAPT REPLIED UNABLE. SHORTLY AFTER LIFT-OFF WE COMMENCED OUR ASSIGNED TURN TO 140 DEGS AND IMMEDIATELY GAINED VISUAL CONTACT WITH CONFLICTING TFC AT VERY CLOSE RANGE; 1 O'CLOCK; AND SLIGHTLY HIGH. CAPT LOWERED NOSE AND WE PASSED BENEATH WITHOUT INCIDENT. CONCERN IS THAT WE DISREGARDED A CTLR INSTRUCTION WITHOUT DECLARING AN EMER AND A LOSS OF SEPARATION RESULTED. SUPPLEMENTAL INFO FROM ACN 329518: ON TKOF ROLL AT ABOUT 100 KTS; FO FLYING; HEARD ACFT XMIT; 'GOING AROUND.' TWR ISSUED GAR INSTRUCTIONS TO THAT ACFT FOLLOWED QUICKLY BY INSTRUCTIONS TO US TO CANCEL TKOF CLRNC. WE WERE NOW APCHING 130 KTS; WT WAS 149500 LBS. I PNF; IMMEDIATELY RESPONDED NEGATIVE; UNABLE; WE'RE GOING DUE TO THE HIGH SPD AND HVY GROSS WT. AT THIS POINT WE WERE AT LIFT-OFF AND ACFT ON GAR ON RWY 04R WAS SIGHTED CLBING ON THE GAR. IT WAS APPARENT THAT WE WOULD BE ABLE TO FLY UNDER AND/OR BEHIND THIS ACFT WHICH WE WERE ABLE TO DO WITH NORMAL BANK AND SLIGHTLY LOWER CLB ANGLE THAN NORMAL. OUR ACFT PASSED ABOUT 1/2 MI BEHIND AND APPROX 400 FT BENEATH THE OTHER ACFT. I DON'T KNOW THAT ANYONE CAN BE FAULTED FOR THIS OCCURRENCE. WE PROBABLY NEED TO LOOK AT INTERSECTING RWY OPS AGAIN. ONE CONCERN I NEED TO ADDRESS IS NOT COMPLYING WITH ATC INSTRUCTIONS TO ABORT THE TKOF. IN THIS SIT MY JUDGEMENT WAS THAT WE WOULD BE IN MORE PERIL BY ABORTING THAN BY CONTINUING TKOF AND MAINTAINING VISUAL SEPARATION.