Here's what rhymes with oncer.
                        
                        
                    
                                Words that almost rhyme with oncer
under  wonder  ulcer  other  upper  utter  thunder  hunter  runner  gunner  usher  udder  sunder  culture  mother  color  cover  summer  suffer  butter  lover  rubber  buffer  rupture  supper  vulgar  molar  plunder  asunder  blunder  butler  comer  cutter  gutter  lustre  muster  rudder  shudder  shutter  hover  luster  mutter  paler  plunger  rougher  sucker  tougher  buzzer  duller  succor  tuber  vulture  bugger  duster  juster  muffler  putter  umber  another  number  brother  younger  hunger  cluster  sulphur  juncture  lumber  sulfur  flutter  puncture  youngster  bumper  bunker  clutter  drummer  tumbler  blubber  bluster  erasure  jumper  plumber  smother  adjuster  crusher  plover  smuggler  sputter  stutter  structure  recover  sculpture  ocular  sculptor  slumber  uncover  ligature  abductor  coadjutor  scrubber  discover  conductor  midsummer  aquaculture  outnumber  premolar  watercolor  embrasure  encumber  pustular  tricolor  agriculture  instructor  restructure  conjuncture  undercover  filibuster  acupuncture  constructor  horticulture  chronometer  counterculture  rediscover  substructure  pentameter  superstructure  cartographer  infrastructure  semiconductor  superconductor