LOSS OF SEPARATION ENDING IN TCASII RA TRAILS POOR CTLR TECHNIQUE AND ATTEMPT TO EVADE RESPONSIBILITY AS ACR MLG APCHS OAK.

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LEVEL AT 7000 FT MSL; ENRTE TO OAK THE CTLR ISSUED INSTRUCTIONS TO DSND TO AND MAINTAIN 6000 FT MSL. THE CLRNC WAS ACKNOWLEDGED AND AS I SET THE NEW ALT INTO THE ALERTER; THE CTLR SIMULTANEOUSLY CAME BACK AND ISSUED ONCOMING TFC TO US. GIVEN THE RANGE AND DISTANCE OF THE ISSUED TFC; IT OCCURRED TO BOTH MYSELF AND THE CAPT; (WHO WAS FLYING THE ACFT) THAT AN ERROR; EITHER IN THE PREVIOUSLY ISSUED CLRNC OR IN THE CONFLICTING ACFT'S POS MAY HAVE OCCURRED. DUE TO THE CLOSE PROX OF THE ISSUED TFC; I IMMEDIATELY DIVERTED MY ATTN OUTSIDE THE ACFT; IN AN ATTEMPT TO ESTABLISH A VISUAL ON THE TARGET. THE CAPT HESITATED; CONSULTED HIS TCASII; (WHICH DID NOT PORTRAY SAID TFC); AND BEGAN A SLOW DSCNT. I TRIED TO RE-ESTABLISH COM WITH THE CTLR TO VERIFY THE PREVIOUS MESSAGE; BUT THE CTLR WAS ENGAGED WITH OTHER ACFT. SIMULTANEOUSLY WITH THE INITIATION OF MY VISUAL SEARCH THE TCASII STARTED SOUNDING OFF WARNINGS REGARDING THE POTENTIAL FOR A HAZARDOUS SIT. THE CAPT ARRESTED THE MIN DSCNT HE HAD INITIATED AND CLBED SLIGHTLY. THE CTLR THEN ADMONISHED US FOR BEGINNING OUR DSCNT OUT OF 7000 FT; SAYING SHE HAD TOLD US TO MAINTAIN 7000 FT WHEN SHE ISSUED THE TFC. IT WAS DURING THIS LAST VERBAL EXCHANGE THAT WE GAINED A VISUAL ON THE CONFLICTING ACFT PASSING BENEATH US. THIS WHOLE EVENT HAPPENED IN WHAT SEEMED LIKE A NANOSECOND. THE WX WAS CLR AND VISIBILITY EXCELLENT. RADIO TFC WAS EXTREMELY HVY AND QUITE POSSIBLY THE CTLR'S RETORT OF THE ORIGINAL CLRNC TO 6000 FT WAS BLOCKED. I DO KNOW THAT NO ONE ON THE FLT DECK CONSCIOUSLY ACKNOWLEDGED IT. THE CTLR WAS RELIEVED FROM THE POS PRIOR TO OUR LEAVING THE SECTOR. UPON LNDG; THE CAPT CALLED THE ATC FACILITY AND WAS TOLD SOMETHING TO THE EFFECT THAT THE MISCOM WAS NOT OUR PROB. HE WAS TOLD; MANY VFR TARGETS TRANSIT THE AREA AND THE POTENTIAL FOR LIKE INCIDENTS IS UNFORTUNATELY COMMON. I DO NOT HAVE THE BIG PICTURE REGARDING COMMAND AND CTL OF THIS AIRSPACE; BUT IF THIS IS TRULY THE CASE; SOME RE-ALLOCATION OF AIRSPACE AROUND OAK OR ADDITIONAL FREQS TO REDUCE RADIO CONGESTION SEEM IN ORDER. ADVANCES IN TECHNOLOGY SUCH AS TCASII; WHICH IDENTED THE CONFLICT POTENTIAL BEFORE WE HAD A VISUAL; ARE NOT FOOL PROOF. OUR RATE OF CLOSURE; THE ANGLE AT WHICH WE CLOSED; COUPLED WITH THE RAPID FIRE DELIVERY OF ATC INSTRUCTIONS TO DIGEST AND THE INABILITY TO GET AN IMMEDIATE VERIFICATION OF ATC'S LAST COM; COULD HAVE MADE THIS SIT MORE SIGNIFICANT. THE HUMAN FACTORS AND WORKLOAD SIDE OF THE EQUATION MUST BE EXAMINED; SO PLTS AND CTLRS ARE NOT FACED WITH SIMILAR COMPROMISES.