A300 FLT CREW HAS A RWY EXCURSION AT ORD.

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APCHING CHICAGO O'HARE; PLANNED RWY 9R APCH. ON DOWNWIND RWY 9R WAS ANNOUNCED CLOSED. WE WERE VECTORED FOR AN APCH TO RWY 14R. WINDS WERE 040 DEGS/20 KTS. CEILING BROKEN 070 WITH SN. WHILE WE WERE ON FINAL APCH A BOEING 777 LANDED AND RPTED BRAKING ACTION AS 'POOR.' WE PERFORMED A MISSED APCH. AFTER HOLDING WE WERE OFFERED RWY 4R (WINDS INCREASED AND EXCEED OPERATIONAL LIMITS FOR RWY 14R). WE ACCEPTED. WHILE ON DOWNWIND THE CTLR RPTED THE RWY 4R WAS NOW CLOSED AND WOULD NOT OPEN; AGAIN. INFORMED THAT RWY 9R WAS NOW OPEN AND 'PLOWED AND DEICED' WITH MU34. THE MU COMPUTED TO FAIR BRAKING IN OUR MANUAL. WE WERE BARELY LEGAL WITH WINDS 030 DEGS/21 KTS GUSTING 28 KTS. ELECTED TO PERFORM A MONITORED CAT 1 APCH TO RWY 9R; FLAPS 20 DEGS. AT 100 FT ABOVE MINIMUMS; I ANNOUNCED I HAVE THE AIRPLANE. AT 100 FT AGL I PUT IN XWIND CTLS. WE WERE ABOUT 8 FT R OF CTRLINE JUST PRIOR TO TOUCHDOWN; BUT TRACKING STRAIGHT DOWN THE RWY. LANDED AT A NORMAL POINT IN THE TOUCHDOWN ZONE; ON SPD. 'MED' AUTO-BRAKES HAD BEEN SELECTED PRIOR TO LNDG. MAX REVERSERS USED AFTER LNDG. EVERYTHING WAS NORMAL AND WE WERE ON CTRLINE APCHING 80 KTS. THEN THE AFT END OF THE AIRPLANE BEGAN SLIDING R AND THE NOSE YAWED TOWARD THE WIND (L). THE AIRPLANE BEGAN SLIDING L. I USED RUDDER; DIFFERENTIAL REVERSE; AND AT SOME POINT AFTER PRESSING BRAKES HARD ENOUGH DIFFERENTIAL BRAKING AND NOSEWHEEL STEERING; BUT THE LEFTWARD SLIDE CONTINUED. BRAKING SEEMED CLOSE TO NIL. AIRPLANE STOPPED 70 FT W OF THE TXWY M6 SIGN; WITH THE L MAIN WHEELS OFF THE RWY AND THE NOSEWHEELS BARELY OFF; AND THE R MAINS ON THE RWY. AIRPLANE WAS COCKED 10 DEGS TO 20 DEGS L.