In Japanese writing, the kana お (hiragana) and オ (katakana) occupy the fifth place, between え and か, in the modern Gojūon (五十音) system of collating kana.

In the Iroha, they occupy the 27th, between の and く.

お and オ originate, via man'yōgana, from the kanji 於. Scaled-down versions of the kana (ぉ, ォ) are used to express morae foreign to the Japanese language, such as フォ (fo).

The hiragana お is made with three strokes: The katakana オ is made with three strokes:

* When lengthening "-o" morae in Japanese braille, a chōon is always used, as in standard katakana usage instead of adding an お / オ.