It travelled further than any other element of Vedic religion, and each tradition that took it up supplied its own account of what it holds.
Buddhism uses it at the head of a great many mantras, most famously the six-syllable formula of Avalokiteshvara, and Vajrayana pairs it with two further syllables as a triad addressed to body, speech and mind. Buddhist commentary does not accept the Upanishadic identification with an unchanging self, since the doctrine of not-self rules that out, and reads the syllable instead in terms of the enlightened body or of the whole of sound.
Jainism reads it as an acronym. The five syllables of the great homage formula, arihant, ashiri or siddha, acharya, upadhyaya, muni, are taken to compress into the one sound, so that uttering it salutes all five categories of the venerable at once. This is a specifically Jain derivation and it is not a variant of the Hindu one.
Sikhism opens its scripture with Ik Onkar, one Onkar, and the numeral is doing the work: the point being made is oneness, and the syllable is inflected into a statement about a single formless creator rather than left as a bare sound.
What none of this is, is a shared meaning. Four traditions use the same sound and give four incompatible accounts of it, which is a good illustration of how a formula can be transmitted intact while what it is understood to say changes completely. The reduction of it to a generic sound of the universe in twentieth-century Western usage flattens all four.