Devanagari O ओ

O is the thirteenth letter of the varnamala, the combination vowel that results when a and u come together, and the counterpart on the labial side to what e is on the palatal. Like the other three sandhyakshara it is always long and always heavy in metre, and it has no short partner in the series. Its vowel sign is compound, an upright stroke set to the right of the consonant together with a stroke above the headline, so it borrows from the way long a is written and from the way e is written and combines the two. The letter has more visibility in religious usage than any other combination vowel because Om is written with it. The syllable is a and u combined into o, with the nasal closing it, which is precisely why the tradition that takes Om apart into three sounds is able to do so: the analysis into a, u and m is a reading of the syllable back through the ordinary rule of sandhi that produced it.

The Mandukya Upanishad is the text that develops that analysis furthest, correlating a with waking, u with dream and m with deep sleep, and the silence beyond them with a fourth state it names only as the fourth. That scheme belongs to the Upanishad and to its long tradition of commentary, and it is recorded here as theirs.

Away from Om, the letter behaves as an ordinary product of the strengthening ladder and of sandhi. It is the first strengthening of u, standing to that vowel as e stands to i, and it therefore appears throughout derived vocabulary built on roots with a labial vowel. It also arises at word joins wherever a word ending in a meets a word beginning with u, so a recited passage shows it where the separated words had no such vowel.

The rules are stated in Panini's grammar in the compressed form that the Shivasutras make possible, and they apply inside words and across word boundaries alike. It is worth saying plainly what this atlas does not assert for the letter. Sanskrit does not assign individual numerical values to its letters in the way Hebrew does under gematria or Greek under isopsephy, and no such value is claimed here. Indian traditions do have systems that convert letters into numbers, the katapayadi scheme used by astronomers and by musicians being the best known, but that scheme assigns values to consonants by their position in the vargas and treats a vowel standing alone as carrying none; it is a separate subject from the varnamala and would need its own sourcing. What is recorded for o is its place in the series, its articulation and length, its role in the written form of Om, and its membership of the fifty-letter garland the Tantric traditions count.