NMAC IN IFR CONDITIONS ON LOC TO RWY 9R AT CHICAGO O'HARE ARPT.

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THE CAPT WAS FLYING THE ACFT AND I WAS THE PNF. THE ATIS WAS ADVERTISING APCHS TO BOTH RWY 9L&R; AS WELL AS RWY 4R. THE WX WAS AS OF THE XA56Z RPT; WIND 020 DEGS AT 16 KTS WITH GUSTS TO 23 KTS; 8 SM VISIBILITY; CEILING 1500 FT BROKEN; 4300 FT BROKEN; FL200 BROKEN. WE HAD BEEN CLRED FOR THE APCH AND HAD THE ACFT IN THE LNDG CONFIGN WITH THE FLAPS AT 25 OR 30 DEGS AND WERE AT ABOUT 160 KTS. WE WERE SLIGHTLY TO THE R (DOWNWIND) OF THE LOC AND SLIGHTLY ABOVE; IF NOT ON THE GS. WE WERE FORCED TO CONFIGURE EARLY IN AN EFFORT TO LOSE ALT AS WE HAD BEEN GIVEN A CLOSER BASE TURN THAN EXPECTED. ONCE ESTABLISHED ON THE APCH; I NOTICED A TCASII TARGET AT ABOUT OUR 10 O'CLOCK POS AND 2-3 MI; INDICATING 1000 FT BELOW OUR ALT AND IN LEVEL FLT. INITIALLY; IT APPEARED TO BE ON A PARALLEL APCH FOR RWY 9L; BUT IT LOOKED CLOSER THAN NORMAL AS IF IT HAD BEEN BLOWN DOWNWIND OF THE RWY 9L LOC. AS WE CONTINUED THE APCH; THE TARGET CONTINUED TO GET CLOSER BOTH LATERALLY AND VERTICALLY WITH NO RELATIVE BEARING CHANGE. THE TARGET APPEARED TO BE MOVING DUE S AT A SLOW RATE. I CALLED OUT THE TFC TO THE CAPT. AT THIS POINT; WE WERE JUST OUTSIDE DEANA AT ABOUT 2300-2400 FT. RIGHT AWAY THE TCASII SOUNDED OFF A TA; FOLLOWED ALMOST IMMEDIATELY BY A CLB RA. THE VERT SPD INDICATOR LIGHTS WERE CALLING FOR A CLB RATE OF APPROX 2000 FPM. THE CAPT INITIATED AN AGGRESSIVE AVOIDANCE MANEUVER; FOLLOWING THE GUIDANCE OF THE TCASII. AT THE CLOSEST POINT; THE RA SYMBOL OF THE OTHER ACFT APPEARED TO TOUCH OUR ACFT SYMBOL AND THE ALT MARKER INDICATED THAT IT WAS ONLY 300 FT BELOW US AND LEVEL. WE WERE IMC AND NEVER ACQUIRED THE TFC VISUALLY. I RPTED TO APCH CTL THAT WE WERE RESPONDING TO AN RA. HE ASKED IF WE WERE GOING TO REJOIN THE APCH OR GO MISSED. I TOLD HIM WE WERE MAKING A MISSED APCH. HE TOLD US TO TRACK THE LOC AND CONTACT THE TWR AS WE WERE INSIDE THE MARKER AT THIS POINT. WE CONTACTED THE TWR AND WERE ISSUED MISSED APCH INSTRUCTIONS. INITIALLY; THE TWR ASSIGNED US 3000 FT; BUT THEN ASSIGNED US 4000 FT AS WE WERE ALREADY THROUGH 3000 FT ON THE MISSED APCH. WE WERE THEN SWITCHED BACK TO ANOTHER APCH CTLR FOR VECTORS BACK TO RWY 4R. AFTER LNDG; THE CAPT CALLED THE TRACON SUPVR AND DISCUSSED THE INCIDENT WITH HIM. THE SUPVR STATED THAT THEY COULDN'T IDENT THE OTHER ACFT OR WHERE IT WAS GOING; BUT THAT IT WAS CLR OF THE CLASS B AIRSPACE WHICH; ACCORDING TO THE CHICAGO AB CHART; MEANS THAT IT WAS OUTSIDE THE ORD 6 DME ARC AND BELOW 1900 FT MSL. NO EXPLANATION WAS GIVEN AS TO HOW WE GOT SO CLOSE TO THE OTHER ACFT WITHOUT ANY WARNING FROM THE APCH CTLR. IT IS DIFFICULT TO SAY HOW THIS INCIDENT OCCURRED OR HOW IT COULD HAVE BEEN PREVENTED. THE INVESTIGATION IS ONGOING AND A FINAL RESOLUTION HAS YET TO BE REACHED; SO I SAY THIS ONLY IN SPECULATION. IT APPEARS THAT PERHAPS THERE WAS A BREAKDOWN IN RESPONSIBILITY FOR CALLING CONFLICTING TFC. THE CONFLICT OCCURRED JUST OUTSIDE THE OM FOR RWY 9R. THE APCH CTLR HAD ALREADY CLRED US FOR THE APCH AND HAD TOLD US TO SWITCH TO THE TWR AT THE OM. WE HADN'T SPOKEN TO HIM IN ABOUT 1 MIN. IT'S POSSIBLE THAT THE APCH CTLR THOUGHT WE HAD ALREADY SWITCHED TO THE TWR AND THE TWR CTLR COULD WARN US. THE ONLY THING ABOUT THIS INCIDENT THAT IS CLR IS THAT WE CAME WITHIN 300 FT OF ANOTHER ACFT. WHEN OPERATING IN THE VICINITY OF AN ARPT; PAY PARTICULAR ATTN TO THE TFC DISPLAYED AROUND YOU. USE THE 5 NM SCALE WHEN POSSIBLE TO GIVE THE MOST EASILY INTERPED INFO ABOUT OTHER TFC.