MD80 FLC HAS TCASII RA DURING ARR TO ORD.

Narrative

I WAS FLYING AND CAPT HAD CHKED ON WITH ORD APCH. WE WERE CLRED DIRECT ORD AT 250 KTS. TCASII INDICATED A TARGET AT OUR 1 O'CLOCK POS 1000 FT BELOW AND CLBING. CAPT NOTED THE TARGET WAS NOW 500 FT BELOW AS ATC SAID THAT HE WAS NOT TALKING TO THAT ACFT. TCASII GAVE US A CLB RA; AND I BEGAN TO COMPLY AS CAPT NOTIFIED ATC THAT WE WERE RESPONDING AND IN A CLB. ATC EMPHATICALLY TOLD US TO DSND TO 7000 FT; AND THE TCASII THEN COMMANDED A DSCNT RA. I BEGAN DSCNT AT 8500 FT TO GREEN ARC ON VSI. CAPT SPOTTED TFC DIRECTLY OVERHEAD APPROX 700 FT ABOVE AS WE DSNDED THROUGH 8000 FT. CAPT ASKED ATC WHO WAS TALKING TO THE TFC; AND HE SAID THEY WERE ON DEP FREQ AND THAT HE WOULD CHK INTO IT. HAD PLT ON THE JUMPSEAT AND ALL 3 OF US AGREED THAT THE INITIAL RA WAS TO CLB; FOLLOWED BY A DSCNT RA PRIOR TO THE CLR OF CONFLICT MESSAGE. THE FLT ATTENDANTS WERE SEATED AND THERE WERE NO INJURIES. AT THE TIME OF THE INITIAL RA AND THROUGHOUT THE INCIDENT; WE WERE IMC. WE FLEW ILS RWY 9R AND LANDED WITHOUT FURTHER INCIDENT.