So the word “om” has had sacred meaning in some later Buddhist traditions, particularly the esoteric varieties of Buddhism (as found in Tibet, for example).
In these traditions, invoking om also brings into the practitioner’s self the understanding of unity with the hidden reality of the universe (revealed in the lotus), materially manifested as a Buddhist deity (such as a bodhisattva).
In this sense, esoteric (tantric) Buddhists are following a conceptually similar aim of union btween the self with the cosmos, that we saw in the ātman-Brahman unity of early Hinduism.
In early Buddhism, om was just a way of concentrating the mind (for personal training for enlightenment). Only much later (around the 7th century CE) did the chanting of om take on its mystical association with creating good merit and union with a kind of divinized Buddhist reality.