TCAS RA HELPS AVERT A NMAC WITH A LIGHT TWIN FOR MD90 DEPARTING SNA.

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WE DEPARTED RWY 19R AT SNA. ENRTE TO ZZZ. WX WAS CLR WITH GOOD VISIBILITY. WE WERE DEPARTING ON THE MUSEL 6 DEP THEN VECTORED R TO 230 DEGS; THEN 280 DEGS; WITH A CLB CLRNC TO 8000 FT; OVER THE SAN PEDRO CHANNEL; W OF SANTA ANA AND SW OF SEAL BEACH. THE SOCAL DEP CTLR HAD BEEN CALLING OUT NUMEROUS TFC LOCATIONS -- MOST OF WHICH WE DID NOT SEE AT THE TIME OF CALLOUT. THE CAPT WAS FLYING THE ACFT WITH THE AUTOPLT ENGAGED AT 250 KTS. BOTH PLTS AND THE NON-REVENUE JUMPSEAT LINE CHK AIRMAN SCANNED FOR TFC VISUALLY AND ON THE RADAR SCOPE. WHILE CONTINUING TO TURN TO A 280 DEG HDG; AND PASSING 7000 FT TO 7400 FT CLBING TO 8000 FT; OUR PLANE'S TCAS SYS ISSUED A DSND RA. SHORTLY THEREAFTER; THE CAPT SAW A TWIN ENG; LIGHT PROP ACFT FLYING TOWARD US AND ABOVE; WHILE I ECHOED THE RA TO DSND. I OBSERVED THE ACFT HIGH IN HIS FORWARD WINDSCREEN SHORTLY THEREAFTER. THE CAPT DISCONNECTED THE AUTOPLT AND SMOOTHLY BUT DECISIVELY DSNDED. MEANWHILE; I RADIOED TO THE CTLR THAT WE WERE DSNDING IN RESPONSE TO AN RA. ALL 3 PLTS SURMISED THAT A COLLISION WOULD HAVE OCCURRED HAD WE NOT TAKEN IMMEDIATE EVASIVE ACTION IN RESPONSE TO THE TCAS RA. WHEN THE LIGHT PLANE APCHED OVERHEAD; I ESTIMATED ABOUT A 500 FT VERT CLRNC EXISTED. WE COULD NOT DISCERN IF THE LIGHT ACFT TOOK ANY EVASIVE ACTION. WE BELIEVE IT DID NOT; NOR DID WE HEAR ANY RADIO XMISSIONS FROM THE ACFT. WHEN THE TCAS RA TERMINATED; OUR ACFT WAS ABOUT 7000 FT; I COORD A CLB BACK TO 8000 FT AND DISCUSSED THIS CLOSE CALL SITUATION WITH THE CTLR. AFTER AFFIRMING OUR CLRNC TO RESUME CLB BACK UP TO 8000 FT; THE CONVERSATION WENT SOMETHING LIKE THIS (NOT VERBATIM BUT CLOSE). FO: 'WE HAD AN RA FOR TFC GOING OVERHEAD AT 8000 FT.' CTLR: 'I SHOW TFC BEHIND YOU AT 8000 FT.' FO: 'YES; BUT YOU CLRED US TO 8000 FT.' CTLR: 'ROGER.' THE REST OF THE FLT WAS UNEVENTFUL; ALTHOUGH THE PLTS WENT THROUGH EMOTIONS OF INITIAL SHOCK; THEN FEAR OF COLLISION; THEN AGGRAVATION OVER WHAT JUST HAPPENED AND THE CTLR'S SEEMINGLY NONCHALANCE ABOUT A NEAR ACCIDENT; AND THEN GRATEFULNESS FOR THE TCAS RA WHICH AVERTED DISASTER. AFTER THE FLT; THE FLT ATTENDANTS RPTED THAT THEY AND THE PAX COULD DEFINITELY TELL THE ACFT WAS BEING MANEUVERED UNUSUALLY DURING THE EVASIVE MANEUVER BECAUSE THEY FELT LIGHT IN THEIR SEATS. WE DON'T KNOW IF THE OTHER ACFT HAD A CLRNC FOR THE ALT HE WAS FLYING OR IF HE WAS TALKING WITH ANYONE. WE JUST KNOW THE OTHER ACFT WAS IN THE AIRSPACE WHERE WE WERE CLRED.