BE40 CREW AND A C182 HAD A LESS THAN LEGAL SEPARATION IN ZOA CLASS E AIRSPACE.

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A CESSNA 182 FULL OF SKYDIVERS WAS CLRED FOR TKOF ON RWY 19 BY STS TWR. APPROX 5 MINS LATER; WE WERE CLRED FOR FREES 5 DEP. WE WERE CLRED TO TAKE OFF ON RWY 19. WE WERE CLRED TO 3000 FT. AFTER TKOF; WE CONTACTED ZOA. WE WERE IN RADAR CONTACT. CTLR CLRED US TO 4000 FT. WE RECEIVED A TCASII RESOLUTION OF AN ACFT AT 12 O'CLOCK POS; 400 FT ABOVE US. WE WERE AT 2600 FT MSL CLBING. WE RECEIVED A CLB ADVISORY AND REACTED BY TURNING 45 DEGS TO L AND CLBED. IF WE HAD NOT REACTED; WE WOULD HAVE HIT THE C182 FULL OF SKYDIVERS. UNKNOWN TO US AND NOT CALLED OUT BY THE CTLR WAS A PIPER SEMINOLE. BY REACTING TO OUR TCASII CALLOUTS; WE ALMOST HIT ANOTHER ACFT. ZOA DID NOT ADVISE US OF THIS ACFT UNTIL THEY LOST RADAR SEPARATION. THIS ACFT WAS ABOVE US. AS WE REACTED TO THE FIRST SIT; WE WERE RECEIVING CALLOUTS TO DSND. SIMULTANEOUSLY; CTR WAS TALKING TO EVERYONE; BUT DID NOT PROVIDE ADEQUATE SEPARATION BTWN IFR AND VFR TFC. ALSO; WHY WAS THE C182 FLYING EXACTLY ON THE FREES 5 DEP? THIS COULD HAVE BEEN AVOIDED IF CTR WOULD HAVE VECTORED US S OF STS. TFC CTLR WAS OVERLOADED WITH VFR ACFT.