- Olcott, Henry Steel, his Buddhist Catechism, i: [265].
- Old Semicolon, nickname given to Hearn, i: [50].
- Omar, Caliph, i: [281].
- Omiki dokkuri no kuchi-sashi, form of, ii: [80].
- Ōnamuji-no-Mikoto, Japanese deity, ii: [9].
- Opposition, value of, ii: [406].