o-

combining form

variants or oo-

: egg

oology

specifically : ovum

oogonium

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Words Starting With O

Browse 9,760 words and phrases that start with the letter O, from oaf to ozotype.

Word History

Etymology

Greek ōi-, ōio-, from ōion — more at egg entry 1

Kids Definition

o

1 of 2 noun

ˈō

often capitalized

1

: the 15th letter of the English alphabet

2

: zero entry 1 sense 1

O

2 of 2

variant of oh entry 1

Medical Definition

o-

1 of 4 abbreviation

orth-; ortho-

O

2 of 4 noun

ˈō 

: the one of the four ABO blood groups characterized by the absence of antigens designated by the letters A and B and by the presence of antibodies against these antigens

O

3 of 4 abbreviation

1

opening

2

[Latin octarius] pint

—used in writing prescriptions

O

4 of 4 symbol

oxygen