-UM WORDS THAT CAN ALSO BE SPELLED -ON

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Words that can be spelled with either the Greek -ON or the Latin -UM:

ALLUVIUM
CARDAMUM
DILUVIUM
ELYTRUM
FANUM
GONIUM
MEZEREUM
ODEUM
ORGANUM
PHYLUM
PLASTRUM
PLECTRUM

Words that mean just about the same thing but are not exactly 
identical, at least not according to the OSPD:

DOMINIUM
SOLATIUM

Words too long for the OSPD, but which almost certainly mean the same
with either ending:

CONTAGIUM
CRITERIUM
EPITHALAMIUM
PROTHALAMIUM

Words that are still good if you change UM to ON but mean something
totally different:

CHUM
CUM
ELECTRUM
FERMIUM
HUM
MINIUM
MUM
NEUM
ONIUM
SOLUM
SUM
TRILLIUM
UM
YUM