FLT CREW DESCENDS EARLY ON THE WESTERN ARC OF THE VOR DME B APPROACH TO MSO. ERROR IS CAUGHT BY ATC.

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WE WERE DSNDING INTO MISSOULA AND HAD BEEN CLRED VIA THE MULLAN PASS TRANSITION TO THE LOC FOR RWY 11. WE WERE SET UP FOR THE ILS 11 APCH AND BRIEFED IT EARLIER. WHEN WE CHKED IN WITH APCH WE WERE GIVEN A HDG FOR VECTORS TO THE VOR-DME B APCH TO RWY 29; AND FURTHER DSCNT TO 12000 FT. I QUICKLY REPROGRAMMED THE FMS FOR THE VOR-DME B APCH AND THE FO (PF) BEGAN BRIEFING THE NEW APCH. I ASKED THE CTLR IF WE COULD EXPECT VECTORS OR THE DME ARC. SHE RESPONDED THAT EITHER WOULD BE FINE AND THAT THE ARC WOULD ALLOW US TO GET LOWER THAN SHE COULD VECTOR US. SINCE IT LOOKED LIKE WE WERE BEING VECTORED TOWARD THE STEVI INTXN AND THE ARC WAS ALREADY PROGRAMMED IN THE FMS; I ASKED IF WE COULD GO DIRECT OCEVO THE IAF APPROX 5 MI AHEAD OF OUR POS. SHE CLRED US DIRECT OCEVO; MAINTAIN 12000 FT UNTIL ESTABLISHED ON THE ARC; CLRED FOR THE APCH. WE PROCEEDED DIRECT OCEVO AND BEGAN TO FINISH THE APCH BRIEFING. APPROX 5-8 MI AFTER XING OCEVO; THE CTLR SAID THAT HER APCH PLATE INDICATED THAT WE SHOULD BE AT 12000 FT. I LOOKED DOWN AT THE APCH PLATE AND IT DID INDICATE THAT WE SHOULD BE AT 12000 FT; WE WERE LEVEL AT 10000 FT. WE IMMEDIATELY CLBED TO 12000 FT AT THE SAME TIME THE CTLR ISSUED A CLB CLRNC. WE WERE VMC WITH CLOUDS BELOW US OBSCURING THE TERRAIN AND DID NOT RECEIVE A GND PROX WARNING. THE CTLR CONFIRMED THAT WE WERE TO BE AT 12000 FT UNTIL XING THE 173 DEG RADIAL AT WHICH TIME WE COULD DSND TO 9800 FT AND CONTINUE THE APCH AND TOLD US TO CONTACT TWR. I CONCURRED; THANKED HER FOR HER HELP AND CONTACTED TWR. WE LANDED WITHOUT INCIDENT. HERE IS WHAT I BELIEVE TO BE SOME OF THE CONTRIBUTING FACTORS THAT CAUSED US TO END UP LOW ON THE ARC. WHEN WE CROSSED THE IAF OCEVO; I MAY HAVE REFED THE OTHER IAF ON THE APCH CHART THAT HAS A VERY SIMILAR NAME OF OCEDA INSTEAD OF OCEVO. THE MINIMUM ALT AFTER OCEDA IS 10000 FT; WHILE THE MINIMUM ALT AFTER OCEVO IS 12000 FT. WE WERE GIVEN A LAST MIN CHANGE TO OUR PLANNED APCH WHICH CAUSED US TO DO SOME LAST MIN PROGRAMMING OF THE FMS AND BRIEFING OF THE APCH. THE LAST MIN CHANGES TO THE APCH; THE APCH BRIEFING; MY MONITORING THE FO WHO WAS FLYING THE APCH; AND THE SIMILAR NAMED INITIAL APCH FIXES MAY HAVE CAUSED ENOUGH OF A DISTR FOR ME TO REF THE WRONG ALT ON THE CHART. SUPPLEMENTAL INFO FROM ACN 778461: WE WERE STARTING OUR DSCNT INTO MISSOULA; LEAVING AROUND 15000 FT; SET UP FOR THE NAPTN ILS RWY 11 APCH. I AM ON IOE. IT STILL TAKES ME A LITTLE LONGER THAN NORMAL TO SET UP THE APCH IN THE FMS. THE WINDS FAVORED RWY 29 SO WE ASKED FOR THE VOR DME OR GPS-B APCH. ATC CLRED US FOR THE APCH AND SENT US DIRECT TO OCEVO TO JOIN THE 16 DME ARC. THAT'S WHERE THE PROB HAPPENED. WE CONFUSED OCEVO FOR OCEDA ON THE APCH PLATE. WE JOINED THE ARC AT 12000 FT; AND WHEN WE WERE ESTABLISHED; MY CAPT DIALED IN 10000 FT AND WE STARTED DSNDING DOWN TO 10000 FT. WE WERE IN VMC AND COULD SEE THE TERRAIN. ATC CALLED US WHEN WE WERE AT AROUND 10900 FT AND TOLD US WE WERE IN A LOW TERRAIN AREA AND NEEDED TO STOP DSNDING AND CLB IMMEDIATELY BACK UP TO 12000 FT. WE DID THAT AND NO FURTHER WORDS OR ACTIONS WERE TAKEN. THIS EVENT OCCURRED BECAUSE I WAS REPROGRAMMING THE FMS FOR THE NEW APCH. MY CAPT WAS WATCHING ME MAKING SURE I WAS FOLLOWING THE RIGHT PROCS SINCE I AM ON IOE. THIS WAS THE FIRST APCH I'VE DONE THAT'S INVOLVED AN ARC SO IT PROBABLY TOOK ME A LITTLE LONGER THAN USUAL TO MAKE SURE EVERYTHING WAS SET UP CORRECTLY. THAT'S WHERE WE CONFUSED OCEVO WITH OCEDA.