The Greek Small Letter Omicron with Tonos symbol (Ο). Copy and paste it, or use its Unicode, HTML, and CSS codes.
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The quickest way is to copy Ο from this page and paste it anywhere. You can also insert it with its Unicode, HTML, or alt code listed above.
Ο is U+03CC (decimal 972).
Use ό in HTML to render Ο on a web page.
On Windows you can type it with Alt+972 (hold Alt and enter the number on the numeric keypad).
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