A LNDG B737 ON RWY 27L; FLEW DIRECTLY OVER THE TOP OF AN MD80 ON TKOF ON RWY 22L AT ORD.

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LOSS OF SEPARATION BTWN TKOF ON RWY 22L AND LNDG RWY 27L. I WAS THE FO ON THIS FLT AND ALSO THE PF. FLT XXX WAS GIVEN CLRNC FOR TKOF ON RWY 22L AT ORD AND TO FLY THE RWY HEADING. I READ BACK THE CLRNC AND PROCEEDED WITH FINISHING THE MECHANICAL CHKLIST. AFTER CALLING THE 'BEFORE TKOF CHKLIST' COMPLETE I LOOKED OUTSIDE TO OUR ACFT'S 10 O'CLOCK POS; AND SAW A B737 ON FINAL TO RWY 27L. THE CAPT ON OUR ACFT WAS BEGINNING TO MAKE THE TURN TO LINE UP WHEN I SAID 'THIS LOOKS TOO CLOSE' HE LOOKED AND AGREED. HE GAVE ME THE THROTTLES AND I IMMEDIATELY STARTED TO SPOOL THE ENGS AND BEGIN THE TKOF ROLL. JUST AS THE ROLL STARTED THE TWR CALLED OUT TO WHAT WE HEARD AS 'ACFT XXX HOLD.' THE CAPT REPLIED 'XXX HOLD' AND HAND SIGNALED FOR ME TO BRING THE THROTTLES BACK AND STOP. WE HAD ROLLED ONLY A FEW FT WHEN WE HEARD THE SOUND OF AN ACFT FLY OVER OUR HEAD AT 70 DEGS AND LAND RWY 27L. TWR THEN SAID; 'ACR XXX GO.' THE CAPT SAID APPROX 'WE HEARD HOLD NOT GO.' THEN I SAID 'TWR WE JUST HAD AN ACFT LAND OVER US ONTO RWY 27L.' TWR RESPONDED WITH APPROX 'YES GO YOU'RE CLRED FOR TKOF RWY 22L.' WE ROLLED AND COMPLETED THE FLT. I FEEL A COUPLE OF THINGS HAPPENED: LOSS OF SAFE SEPARATION; LOSS OF SITUATIONAL AWARENESS BY THE TWR; AND MISUSE OF PROPER PHRASEOLOGY DURING A CRITICAL TIME. IF WE WERE NOT EXPEDITING OUR TKOF FAST ENOUGH THEN TWR SHOULD HAVE SENT THE OTHER ACFT AROUND OR NOT EVEN CLR US FOR TKOF IN THE FIRST PLACE. ONCE ON THE RWY ALL WE SHOULD HEAR IS EXPEDITE THE TKOF OR CANCEL TKOF CLRNC; BECAUSE HOLD AND GO ON A BUSY RADIO COULD SOUND ALIKE. ON OUR END; WE COULD HAVE DEMONSTRATED BETTER JUDGEMENT MAYBE BY LOOKING IMMEDIATELY AND SEE THERE WASN'T ENOUGH SEPARATION AND NOT TAKE OFF.