B737-300 FLT CREW HAS PRESSURIZATION MALFUNCTION; EXECUTE DSCNT.

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AT CRUISE AT FL220 CABIN ALT WARNING HORN SOUNDED. I KNOW; AND I HAVE READ MANY TIMES; THAT CREWS OFTEN MISTAKE IT FOR THE TKOF WARNING HORN. NEVER THOUGHT IT WOULD HAPPEN TO ME. MY INITIAL THOUGHT WAS; 'WHAT IS CAUSING THE TKOF WARNING HORN TO GO OFF.' IT TOOK JUST A FEW SECONDS THOUGHT TO REALIZE THAT IT WAS THE CABIN ALT WARNING HORN. WE PERFORMED THE PROC AND RECEIVED AN IMMEDIATE DSCNT FROM CENTER. NO EMER WAS DECLARED AND WE WERE ABLE TO HOLD THE CABIN IN PACKS HIGH. AT SOME POINT IN THE DSCNT THOUGH; MY FO AND I BOTH REALIZED THAT WE HAD NOT ACTUALLY COMPLETED THE PROC. WE HAD NOT PUT ON OUR OXYGEN MASKS. SUPPLEMENTAL INFO FROM ACN 723002: AT CRUISE ALT JUST PRIOR TO TOP OF DSCNT WE GOT THE TKOF CONFIGN/ALT WARNING HORN. THE CAPT REQUESTED A DSCNT AND WE STARTED RUNNING THE ABNORMAL PRESSURIZATION CHKLIST. THE CABIN ALT WAS IN A SLOW CLB THROUGH 10000 FT AND THE CABIN EMER OXYGEN MASKS HAD NOT DEPLOYED. WE VERIFIED THE PRESSURIZATION SWITCHES AND IN THE DSCNT PUT BOTH PACKS TO HIGH. THIS GOT THE PRESSURIZATION UNDER CTL AND PUT THE CABIN PRESSURE BACK INTO A DSCNT. WE SENT AN ACARS MESSAGE TO DISPATCH THAT WE WOULD NEED MAINT FOR THE PRESSURIZATION PROB. WE DID NOT DECLARE AN EMER WITH ATC BECAUSE WE WERE AT OUR DSCNT AND WE WERE ABLE TO KEEP THE PRESSURIZATION UNDER CTL. WHILE ANALYZING THE MALFUNCTION AND THEN EXECUTING CHKLIST WE DID OMIT THE FIRST 2 STEPS OF THE CHKLIST WHERE WE SHOULD HAVE PUT ON THE OXYGEN MASK. THE SURPRISE OF THE TKOF CONFIGN WARNING HORN AT ALT WITH NO RAPID DECOMPRESSION OR APPARENT PRESSURIZATION PROB LEADS TO TRYING TO ANALYZE THE PROB BEFORE EXECUTING THE FIRST 2 STEPS OF THE CHKLIST. ADDITIONALLY THE PRESSURIZATION CHK AT 10000 FT APPEARED NORMAL BUT WE DID NOT CATCH THAT THE PRESSURIZATION STAYED IN A SLOW CLB.