The Greek Capital Letter Omicron with Tonos symbol (Ξ). Copy and paste it, or use its Unicode, HTML, and CSS codes.
More from Greek Letters.
The quickest way is to copy Ξ from this page and paste it anywhere. You can also insert it with its Unicode, HTML, or alt code listed above.
Ξ is U+038C (decimal 908).
Use Ό in HTML to render Ξ on a web page.
On Windows you can type it with Alt+908 (hold Alt and enter the number on the numeric keypad).
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