ACR MLG ACFT RETURNED TO LAND AFTER THE ACFT CABIN PRESSURIZATION FAILED AND ALL PAX OXYGEN MASKS DROPPED DOWN FOR USE.

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HE HAD JUST TAKEN OFF FROM RNO AND WERE ON A VECTOR TO INTERCEPT THE LOVELOCK 208 DEG RADIAL. WE HAD AN INOP APU AND HAD MADE A PACKS OFF TKOF IAW THE FLT MANUAL. AFTER PWR REDUCTION AFTER TKOF; THE PACKS WERE TURNED ON AND PRESSURIZATION CTLR WAS SET TO AUTO IAW FLT MANUAL PROCS. THE CABIN BEGAN TO PRESSURIZE AND CLBOUT APPEARED NORMAL. PASSING APPROX 12500 FT AN INTERMITTENT WARNING HORN CAME ON. WE SUSPECTED THE HORN WAS CAUSED BY A PROBLEM WITH THE AIR/GND SWITCH ON THE GEAR AND BEGAN TO TROUBLESHOOT. WE CHKED THE GEAR (WENT FROM OFF TO UP WITH THE GEAR HANDLE); FLAP ANNUNCIATORS; AND CYCLED SPD BRAKES IN ITS DETENT. THE CAPT PULLED OUT THE HANDBOOK TO RECONFIRM THE ITEMS ON THE TKOF WARNING SYS. WE CONTINUED OUR CLB AND PASSING 15500 FT WE GOT AN OVERHEAD ANNUNCIATOR LIGHT AND PAX OXYGEN LIGHT. AT THE TIME WE NOTICED THE CABIN ALT WAS BTWN 14000-15000 FT . WE ALSO HAD THE PRESSURIZATION CTLR 'STANDBY' MODE LIGHT ON WITH THE MODE SWITCH IN AUTO. THE CABIN RATE OF CLB WAS STILL CLBING AND THE CAPT CYCLED THE CTLR FROM AUTO TO STANDBY BACK TO AUTO AND THEN BACK TO STANDBY. WE REGAINED CTL OF THE PRESSURIZATION. WE REQUESTED CLRNC BACK DOWN TO 10000 FT WHEN WE GOT THE PAX OXYGEN LIGHT AND REGAINED PRESSURIZATION CTL IN A DSCNT PASSING ABOUT 14000 FT HEADED FOR 10000. THE PAX OXYGEN MASKS DEPLOYED WHEN THE CABIN EXCEEDED 14000 FT. I FEEL WE COULD HAVE PREVENTED THE MASKS FROM DEPLOYING HAD WE RECOGNIZED THE HORN WAS A PRESSURIZATION AND NOT AN AIR/GND SWITCH PROBLEM. BOTH PROBLEMS USE THE SAME WARNING HORN. NEITHER OF US NOTICED ANY PROBLEMS WITH OUR EARS WHEN THE CABIN WAS NOT PRESSURIZING. WE BOTH FELT THIS WOULD BE ONE OF YOUR FIRST CLUES OF A PRESSURIZATION PROBLEM.