DURING A CHANGE IN RWYS AT ORD; THE CTLR ALLOWED OPPOSITE DIRECTION TFC; A B727 AND AN MD80 TO CONFLICT; RESULTING IN AN OPERROR.

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I WAS WORKING E ARR POS AT C90; VECTORING TO RWY 9L. ORD WAS LNDG RWY 9L AND RWY 4R. WX CONDITIONS IMPROVED ENOUGH TO PERMIT APCHS TO RWY 9R AND I WAS INSTRUCTED TO TRANSITION MY TFC FLOW FROM ILS 9L TO ILS 9R. IN DOING SO; I BECAME SLIGHTLY DISTR AND NEGLECTED TO STAGGER ALTS ON OPPOSITE DIRECTION BASE LEG ACFT. THIS WOULD NOT HAVE BEEN A PROB SINCE ONE OF THE ACFT HAD ALREADY BEEN GIVEN A TURN TO INTERCEPT THE LOC. THE PROB/ERROR OCCURRED WITH THAT ACFT MISSING HIS INSTRUCTIONS AND I MISSING THE READBACK ERROR BY ANOTHER ACFT. SUPPLEMENTAL INFO FROM ACN 400896: ON ARR INTO ORD WE WERE ASSIGNED RWY 4R ILS APCH. WHILE BEING VECTORED FO RWY 9R WE WERE GIVEN A 360 DEG HDG AND A CHANGE OF RWY TO 9L. WE WERE ASSIGNED A 120 DEG HDG FOR VECTORS BACK TO RWY 9R. THIS CLRNC WAS ACCEPTED BY ANOTHER ACFT. WE REMAINED ON A 360 DEG HDG AND WAITED FOR A CLR FREQ TO ADVISE ATC ANOTHER ACFT HAD ACCEPTED OUR CLRNC AND WE WERE HDG 360 DEGS AT 5000 FT. WE THEN NOTED A YELLOW TCASII TARGET; SAME ALT; OPPOSITE DIRECTION; AT 12 O'CLOCK POS. WE WERE STILL UNABLE TO COMMUNICATE WITH ATC BECAUSE OF FREQ CONGESTION. ATC THEN ISSUED A DSCNT TO 4000 FT AND A TURN TO 270 DEGS FOR US. WE SIMULTANEOUSLY RECEIVED A TA THEN AN RA. IN RESPONDING TO ATC CLRNC TO 4000 FT; I ALSO ADVISED ATC WE HAD RECEIVED AN RA. ATC RESPONDED WITH AN IMMEDIATE TURN TO 270 DEGS AND DSCND TO 4000 FT. I THEN ADVISED ATC WE WERE COMPLYING AND HAD THE OPPOSITE DIRECTION TFC IN SIGHT. THE RA WAS TERMINATED AND WE WERE VECTORED TO RWY 9R ILS. ILS APCH AND LNDG WERE NORMAL. IN SUMMARY -- FREQ CONGESTION; 3 APCH FREQ CHANGES; 4 DIFFERENT RWY ASSIGNMENTS; AND A READBACK ERROR BY ANOTHER ACFT AT THE WORLD'S BUSIEST ARPT WERE CONTRIBUTING FACTORS TO THIS CONFLICT. AS A FOOTNOTE: 2 WKS EARLIER ON THE SAME LEG WE RECEIVED 4 RWY ASSIGNMENTS ON THE SAME APCH SEQUENCE FROM ORD APCH CTL. THIS IS A RE-SEND OF AN EARLIER RPT.