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Oo- is a combining form used like a prefix meaning “egg.” It is used in many scientific terms, especially in biology.

Oo- comes from the Greek ō��ó��, meaning “egg.” The Greek ō��ó�� helps form the word ō�DZ��ó���Dz�, meaning “egg-bearing” and is the source of the combining form oophoro-, used to indicate the ovary.

Distantly related to the Greek ō��ó�� is the Latin word for egg, ō���ܳ�, source of the related combining forms ov-, ovi-, and ovo-, also used to mean “egg.” The Latin ō���ܳ� is ultimately the origin of ovario-, used, like oophoro-, to mean “ovary.”

What are variants of oo-?

When combined with words or word elements that begin with a vowel, oo- becomes o-, as in oidium.

One example of a scientific term that features the combining form oo- is oocyte, “an immature egg cell of the animal ovary,” including in humans.

The oo- part of the word means “egg,” as we have seen. The second part of the word is -cyte, from the Greek ��ý�ٴDz�, means “cell.” Oocyte literally translates to “egg cell.”

What are some words that use the combining form oo-?

• ooblast
• oogamy
• oogenesis
• oolite (using the equivalent form of oo- in New Latin)
• oology
• oophyte

What are some other forms that oo- may be commonly confused with?

• ob-
• oc-
• opto-
• onco-

Not every word that begins with the exact letters oo- is necessarily using the combining form oo- to denote “egg.” Oodles, ooh, and ooze are three examples. Learn what oodles, ooh, and ooze mean and where they come from at our entries for the words.

The combining form -logy is used to name sciences or bodies of knowledge. What does oology, a branch of ornithology (science of birds), study?