EMB FO RPTS MULTIPLE ACFT FLOWN WITH THE OXYGEN BOTTLE VALVE CLOSED.

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THIS RPT IS NOT SPECIFIC TO A PARTICULAR FLT; SINCE I DID NOT TRACK TAIL NUMBERS/DATES ON THIS ISSUE. RECENTLY; WITHIN THE LAST MONTH OR TWO; I HAVE NOTICED MULTIPLE EMBRAER ACFT; ALL THAT HAD FLOWN 1 OR MORE LEGS THAT DAY; WITH THE CREW OXYGEN BOTTLE VALVE CLOSED. I DIDN'T TRACK SPECIFIC DATES/TAIL NUMBERS BECAUSE AT FIRST; I JUST ASSUMED IT WAS A FLUKE OR CHANCE OCCURRENCE. HOWEVER WHEN I REALIZED HOW OFTEN I SEEMED TO FIND ACFT THIS WAY; I BECAME CONCERNED AND FELT THAT FILING A RPT WOULD BE APPROPRIATE. CAUSE: ALL FLTS HAD ONE THING IN COMMON: THEY HAD SPENT THE PREVIOUS NIGHT AT AN OUTSTATION WHERE OVERNIGHT MAINT HAD OCCURRED. THE MECHS APPARENTLY TURN OFF THE OXYGEN FOR SAFETY; BUT FAIL TO TURN IT BACK ON; AND SOME OF OUR PLTS ARE MISSING THE FACT THAT THEY ARE CLOSED ON PREFLT. ADDITIONALLY; THE MARKINGS ON THE OXYGEN VALVE CAN BE CONFUSING; AND YOU MAY BELIEVE YOU ARE SELECTING THE VALVE'S OPEN POS WHILE ACTUALLY SELECTING THE VALVE'S CLOSED POS. THIS IS VERY TROUBLING; BECAUSE CREWS EXPERIENCING A RAPID CABIN DEPRESSURIZATION WOULD LIKELY NOT FIGURE OUT THAT THE OXYGEN WAS OFF UNTIL IT WAS TOO LATE. ON MY PREFLTS I FOUND THE BOTTLE CLOSED; AFTER THE NORMAL OXYGEN TEST FAILED TO WORK PROPERLY. IF YOU DON'T HOLD THE TEST SLIDER AND BUTTON DOWN LONG ENOUGH; THE RESIDUAL PRESSURE IN THE CLOSED OXYGEN SYS DOES NOT BLEED OUT AND YOU WILL NOT REALIZE THE BOTTLE IS CLOSED.