ཨོཾ།

A general explanation of the syllable oṃ is provided by Rong-zom-pa as follows (RZ1: 9): spyir shes par bya bar [= ba?] ’chad na ni | yi ge a dang u dang ma gsum bsdus pa ni | oṃ yin la yi ge gsum ni lus ngag yid gsum dang lha gsum gyi sa bon yin par bram ze rnams kyi rig byed las kyang grags la | lha gsum sku gsung thugs kyi rang rig yin par yang gsang sngags kyis [= kyi] tshul las grags te | de’i phyir yi ge oṃ ni sku gzugs rdo rje’i rang bzhin yin par ’byung bas spyi’i don du sbyar ba la nyes skyon med do ||.

A rough tentative translation of the passage: 

“If to explain [the significance of oṃ] which is to-be-known (?) in general, it is as follows: The coalesce/collage of three syllables (or perhaps letters) a, u, and m is oṃ. It is also known in the Vedic sources of the Brahmans that the three letters are seeds of the body, speech, and mind, and of the three deities. And also that [these] are rang rig (? [seems to be special usage of the term here]) of the three deities [and] of the Body, Speech, and Mind is known in the Mantranaya (i.e. Vajrayāna/Mantrayāna). Thus, because it occurs [in some authoritative sources] that the syllable oṃ is the Body, characterized by an adamantine nature, there is no fault in associating [with the syllable oṃ] in a general sense.”