FLC MISPROGRAMMED FMS FOR WRONG FIX. CTLR INTERVENED.

Narrative

DURING OUR INITIAL APCH INTO ONT WE RECEIVED THE FOLLOWING CLRNC; PROCEED DIRECT TO THE PETIS NDB; MAINTAIN 4200 FT TO PETIS; CLRED ILS RWY 26L APCH. WHILE MY FO WAS READING BACK THE CLRNC I ENTERED DIRECT 'FF26L' INTO THE FMC AND PROCEEDED TO FLY INBOUND. A FEW MINS LATER APCH CTL CALLED AND SAID WE WERE 4 NM L OF PETIS AND TO TURN R TO A 320 DEG HDG; MAINTAIN 4200 FT. AT THIS POINT I REALIZED THAT I HAD BEEN FLYING DIRECT TO THE OM (FONTA) INSTEAD OF PETIS NDB. WE THEN RECEIVED ANOTHER APCH CLRNC AND CONTINUED UNEVENTFULLY TO THE ARPT. WE WERE IN VFR CONDITIONS THE ENTIRE TIME AND NEVER HAD ANY TFC OR TERRAIN CONFLICTS. THE FMS NAV DATA BASE LISTS THE OM (FONTA) AS FF26L INSTEAD OF JUST FONTA. MOST NDB'S ARE COLLOCATED WITH THE OM; REFERRED TO AS LOM'S. HUMAN FACTORS BEING WHAT THEY ARE; I SAW FF26L AND USED THAT WAYPOINT THINKING AT THAT MOMENT THEY WERE COLLOCATED AND I WAS FLYING TO PETIS. ACTUALLY THEY ARE ABOUT 5 NM APART. IF THE FMS HAD SHOWN FONTA AS THE OM WAYPOINT INSTEAD OF FF26L I THINK IT WOULD HAVE BEEN CLR THAT THEY ARE IN FACT 2 DIFFERENT POINTS. THERE DOESN'T SEEM TO BE ANY CONSISTENCY WITH THE FMS APCH DATA BASE. SOME APCHS SHOW THE OM NAME AND OTHERS USE THE FF (FINAL FIX) FORMAT. THEY SHOULD ALL USE THE CORRECT FIX NAMES. ADDING TO THE CONFUSION THE APCH DATABASE SHOWS PETIS AS SBNB. OUR SOP DICTATES THE PNF MAKE ALL EXECUTABLE ENTRIES TO THE FMS. THIS KEEPS BOTH PLTS 'IN THE LOOP.' I WAS HAND FLYING AND SINCE WE WERE VERY BUSY AT THE TIME; I MADE THE ENTRY TO SAVE TIME. FOLLOWING SOP MAY HAVE PREVENTED THE MIX-UP. FOLLOW SOP AND STAY ALERT; AND PLEASE MAKE THE DATA BASES MORE USER FRIENDLY!