For the better part of three millennia, the Sanskrit mantra OM has been the “sacred syllable” of South Asian religions, central to chanting, ritual, meditation, and yoga. While OM is regarded as primarily a phenomenon of sound and recitation, the syllable has also had a rich career as a written sign and icon. Taking many different forms across the centuries, OM has been inscribed in a wide array of media, from stone to metal, paper to plaster, skin to the subtle body. Notwithstanding OM’s ubiquity in South Asian visual culture, scholarship has seldom addressed the epigraphic and iconographic history of the sacred syllable. What is the oldest inscribed OM? How has the written sign for OM developed across different scripts, regions, periods, and traditions? How have visual representations of OM shaped (and been shaped by) religious doctrines and practices? This paper offers some preliminary answers to these questions by mounting a broad survey of the relevant evidence in material culture; by suggesting a possible trajectory for the emergence of inscribed forms of OM in early medieval South Asia; and by attempting to account for the continued development of OM as a written sign and icon up through the early modern period. I draw a distinction between OM’s orthography—which I argue is governed by the standard conventions of representing Sanskrit sounds in early Indic scripts—and the syllable’s iconography, which I suggest is a somewhat later and derivative development. Moreover, I make the case that Buddhists and Jains made substantial contributions to the emergence of what is often viewed as a quintessentially Hindu symbol.