Ik Onkar opens the Mul Mantar, the foundational statement placed at the very beginning of the Guru Granth Sahib and repeated at the head of its major sections. Written as the numeral 1 followed by the character for oan with an extended stroke, it is the most recognisable Sikh emblem and appears on gurdwaras, publications, vehicles and personal items throughout the Sikh world.
Its meaning is compressed and the compression is deliberate. The numeral does theological work: writing "one" as a digit rather than a word makes the claim unmistakable and difficult to gloss away, and Sikh commentators regularly observe that Guru Nanak chose to begin scripture with a number. Onkar is related to the Sanskrit Om but is not used as it is; the Sikh formulation makes it the name of a single creator rather than a syllable of ritual power, and the emphasis falls on oneness without a second.
The Mul Mantar continues in a sequence of terms that are as much denials as assertions: satnam, whose name is truth; karta purakh, creator being; nirbhau, without fear; nirvair, without enmity; akal murat, timeless in form; ajuni, unborn; saibhang, self-existent; gur prasad, known by the Guru's grace. Without fear and without enmity denies a God who must be placated. Unborn denies incarnation, which distinguishes Sikh theology sharply from the Vaishnava avatar doctrine prevalent in its immediate environment, and the scripture is explicit on the point. Self-existent denies dependence of any kind.
Ik Onkar is not merely a statement of numerical unity. Sikh teaching draws from it a further claim: if God is one and is present in all, then distinctions of caste, gender and religious community carry no ultimate weight. The scripture's statements that the one light is in every heart, and that all are formed of the same clay, are used directly against social hierarchy, and the doctrine and the social critique are presented as a single teaching rather than as an inference from one to the other. Sikh reform movements arguing against caste practice within the community have consistently returned to the Mul Mantar as their ground.