Countable Nouns That Start with O and End with UM

Explore 39 verified English countable nouns starting with O and ending with UM, including Optimum, Opossum, Ovum, Omnium, Omentum, Organum and Ostium. Sort these count nouns with singular and plural forms by word length for ESL grammar practice, determiner usage, and plural spelling below, or review singular and plural count rules in our noun grammar guide.

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Starts with OC
1 words
oceanarium
Starts with OD
2 words
odeum
odontoglossum
Starts with OI
1 words
oidium
Starts with OM5 words
omnium
omentum
omasum
ommatidium
ommateum
Starts with ON3 words
onium
oncidium
onychium
Starts with OO
1 words
oogonium
Starts with OP8 words
optimum
opossum
operculum
oppidum
opusculum
opsonium
ophidiarium
ophthalmophyllum
Starts with OR7 words
organum
origanum
ornithogalum
orarium
organistrum
orontium
ordalium
Starts with OS8 words
ostium
osculum
osmiridium
ostensorium
osteospermum
ossarium
osmeterium
osteoglossum
Starts with OV
1 words
ovum
Starts with OX
2 words
oxonium
oxypetalum

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