A PIPER CHEROKEE PLT ENTERED OAK CLASS C AIRSPACE WITHOUT A CLRNC.

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AFTER LNDG AT METRO OAKLAND ON RWY 27R; I WAS DIRECTED TO BACK-TAXI ON RWY 9L FOR A L DOWNWIND DEP TO THE NW. THE TWR CTLR ASKED IF I WOULD NEED RADAR SVCS MY RETURN FLT TO CCR. I REPLIED THAT I WOULD NOT BE NECESSARY; SINCE I ENCOUNTERED VERY LITTLE GA TFC ALONG THE SAME RTE A FEW MINS EARLIER. ALSO; THE CTLR FOR BAY APCH SEEMED TO BE WORKING SEVERAL LARGE SECTORS AT THE SAME TIME; SO I DIDN'T WANT TO ADD ANY MORE UNNECESSARY WORKLOAD ON HIS SHOULDERS. THE TWR INSTRUCTED ME TO SQUAWK VFR (1200) AND CLRED ME FOR TKOF. AFTER DEP; I CLBED TO THE ALT I HAD FLOWN INBOUND MINS EARLIER -- 2500 FT MSL -- AND ADVISED THE TWR THAT I WANTED TO CIRCLE LAKE MERRITT BEFORE PROCEEDING NW TOWARD RICHMOND; WHICH WAS APPROVED AS REQUESTED. AS I APCHED LAKE MERRITT; THE TWR CTLR ADVISED THAT I WAS LEAVING HIS AIRSPACE AND THAT A FREQ CHANGE WAS APPROVED. IT WAS THEN I REALIZED THAT I HAD JUST ENTERED THE SECONDARY RING FOR THE OAK CLASS C AIRSPACE AT 2500 FT MSL AND WAS NO LONGER IN 2-WAY COM WITH ATC (THE EFFECTIVE ALTS FOR THE RING IN THAT SECTOR WERE FROM 1500 FT MSL TO THE BASE OF THE OVERLYING CLASS B AIRSPACE; WHICH WAS AT 3000 FT MSL). I IMMEDIATELY STARTED A DSCNT TO 1400 FT MSL; WHICH TOOK ABOUT 1 MIN TO COMPLETE. I DECIDED AGAINST ORBITING LAKE MERRITT AND CONTINUED NW UNTIL I HAD FLOWN PAST THE NORTHERN BOUNDARY OF THE OAK CLASS C AIRSPACE; AT WHICH POINT I CLBED BACK TO 2500 FT MSL AND CONTINUED MY FLT. ALTHOUGH I HAD COMMUNICATED WITH BAY APCH ON MY INBOUND LEG; I DID NOT ATTEMPT TO CONTACT THEM THROUGHOUT THIS PERIOD OR AFTERWARD. ALTHOUGH I HAD FLOWN THIS RTE SEVERAL TIMES AT NIGHT; THIS WAS THE FIRST TIME THAT I WAS INSTRUCTED TO SQUAWK VFR BEFORE DEP (BECAUSE I HAD DECLINED RADAR SVCS) AND ALSO THE FIRST TIME 2-WAY RADIO COM WAS SEVERED BY ATC WHILE I WAS STILL WITHIN THE CONFINES OF CLASS C AIRSPACE. (IN THE PAST; I'VE BEEN INSTRUCTED TO CONTACT BAY DEP OR I REMAINED WITH THE LCL CTLR UNTIL PAST THE NORTHERN BOUNDARY OF CLASS C; WHEREUPON I WAS ADVISED THAT 'FREQ CHANGE WAS APPROVED.') IN THE FUTURE; IT IS MY INTENTION TO MORE CAREFULLY DISTINGUISH THE BOUNDARIES OF RESPONSIBILITY BTWN THE APCH/DEP AND LCL CTLRS AND TO FLY ACCORDINGLY. IT IS MY SUGGESTION THAT EVEN THOUGH IT IS NOT THE LCL CTLR'S RESPONSIBILITY TO KEEP VFR TFC FROM VIOLATING CLASS C AIRSPACE IN THE MANNER IN WHICH I DID; HE WAS IN A POS TO CORRECT THE VIOLATION BEFORE IT HAPPENED -- TFC WAS VERY LIGHT; AND I THINK I WAS THE ONLY ACFT HE WAS WORKING AT THE TIME.