CORPORATE ACFT HAS NMAC WITH JUMP PLANE AT 10000 FT. ATC TALKING TO BOTH ACFT.

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ON APCH TO SNA; ZLA HANDED US OFF TO ONT APCH. I ASKED FOR RNAV DIRECT TO KAYOH INTXN. APCH CTLR DECLINED. 2 MINS LATER; TURNED ME R 30 DEGS 'TO AVOID JUMPERS.' 1 MIN LATER EVASIVE ACTION WAS NECESSARY TO AVOID MIDAIR COLLISION WITH A TWIN OTTER; THE JUMP PLANE. I ASKED APCH IF SHE WAS TALKING TO US BOTH. SHE SAID YES. I SAID I WOULD LIKE TO RPT A NEAR COLLISION. I WAS GIVEN ONT APCH TELEPHONE NUMBER FOR SUPVR. AFTER LNDG; I SPOKE WITH THE SUPVR ON DUTY. HE TOLD ME THE CTLR HAD BEEN ON A LANDLINE AND MISSED THE CALL TO US TO AVOID TFC. INFORMED ME HE SENT THE CTLR HOME; RELIVED OF DUTY FOR STRESS. WE DISCUSSED THE TFC PROBS IN THAT AREA. HE SAID HE WAS UNDERSTAFFED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THE SUPVR KNEW EXACTLY WHY HE WAS CALLING WHEN HE DID CALL ON THE GND. SUPVR STATED HE FELT THE NEED TO INCREASE STAFF FOR THAT TIME PERIOD IN THAT SECTOR AND WOULD DO SO. THIS SAME CTLR HAD ANOTHER INCIDENT SHORTLY AFTER THE RPTR'S INCIDENT IN WHICH THERE WAS CLOSE PROX. HE HEARD THE INTERCHANGE ON THE FREQ. HE STATED THE CTLR WAS TRYING TO HANDLE THE SIT AND TALKING AS FAST AS POSSIBLE. SUPVR INDICATED THAT THE TWIN OTTER PLT DID HAVE THE RPTR ACFT IN SIGHT. RPTR SAYS IF SO; HE MUST HAVE BEEN A KAMIKAZE PLT SINCE HE DID NOT APPEAR TO TAKE EVASIVE ACTION. THIS IS THE CLOSEST HE HAS EVER COME TO AN ACTUAL MIDAIR. IF THEY HAD NOT TAKEN EVASIVE ACTION; HE KNOWS THEY WOULD HAVE HIT.