B737 FLT CREW; BEING VECTORED BY NCT TO OAK; EXPERIENCED A CONFLICT AT 3000 FT WITH UNRPTED ACFT.

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THE SECOND ACFT WAS A CESSNA; A SINGLE ENG -- LOOKED LIKE A C172. ON A FERRY FLT TO OAK; I WAS FLYING WITH ONLY THE FLT CREW AND CABIN CREW ON BOARD. WE WERE TALKING TO NORCAL ON 127.2 ON AN ASSIGNED HDG OF 340 DEGS AT 3000 FT MSL APPROX 15 NM S OF OAK. APCH CALLED OUT TFC AT 11 O'CLOCK POS AND 500 FT BELOW (I DO NOT REMEMBER THE DISTANCE). WE WERE LOOKING FOR THE TFC WHEN ALL OF A SUDDEN APCH CALLED TFC ALERT TURN IMMEDIATELY TO THE L 20 DEGS. I TURNED THE AUTOPLT OFF AND USED APPROX 40 DEGS OF BANK TURN L. JUST AS I STARTED THE TURN; THE TCASII WENT OFF WITH AN RA TO DSND. I KEPT THE TURN GOING AND PUSHED THE NOSE DOWN; DSNDING AT APPROX 2000 FPM. GOING THROUGH 2700 FT MSL; WE BOTH SAW THE TFC AT ABOUT OUR 1 O'CLOCK POS AND APPROX 500 FT HORIZ AND 300 FT VERT TO US. AFTER TELLING APCH OF OUR RA HE TOLD US TO DSND TO 2500 FT MSL INTERCEPT THE ILS 29 AT OAK. I KNOW THAT THE AIRSPACE AROUND OAK/SFO IS VERY BUSY AND CONGESTED; BUT KEEPING US BELOW THE CLASS B AIRSPACE GOING INTO OAK IS ASKING FOR TROUBLE. I ALSO BELIEVE THAT HAD I NOT TAKEN THE AUTOPLT OFF AND AGGRESSIVELY BANK AND DSNDED THE ACFT WE WOULD HAVE HIT THE CESSNA.