Countable Nouns That Start with O and End with IUM

Explore 19 verified English countable nouns starting with O and ending with IUM, including Omnium, Ostium, Oceanarium, Onium, Oogonium, Ommatidium and Oncidium. 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 OI
1 words
oidium
Starts with OM
2 words
omnium
ommatidium
Starts with ON3 words
onium
oncidium
onychium
Starts with OO
1 words
oogonium
Starts with OP
2 words
opsonium
ophidiarium
Starts with OR3 words
orarium
orontium
ordalium
Starts with OS5 words
ostium
osmiridium
ostensorium
ossarium
osmeterium
Starts with OX
1 words
oxonium

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