6. Om the Hexad. And then again with a sextuple or Hexaliteral Om composed of the sixth member of Náda (नाद) over and above the aforesaid five parts.
7. Om the Heptad. The septuple Om is described in the Ramatápaniya upanishad as consisting of अ, उ, म, नाद, बिन्दु, शक्ति and नम । Weber’s A. S. Lit. p. 312.
8. Om the Octad. This consists of the aforesaid seven parts, which together with Sánti called in Persian Sákat complete the number, Weber. Id. p. 315.
IV. The Different Denominations of Om.
1. The Initial of the Veda. The om is denominated the heading of the Veda (बेदशिरः) as the Gáyatrí hymn is termed to be its parent (बेदमाता). It stands at the top (शिरस्) of every book (मण्डल), chapter (अष्टक), and hymn (ऋच्) of every Veda either alone by itself or two or three oms put together, as ओमग्निमीले on ignem aiede of the Rigveda, ॐ त्रिसप्ता On triseptem &c. of the Atharvan; again ॐ ॐ ॐ ऋग्वेदाय स्वाहा on I salute thee O Rigveda &c.
It is hence used at the head (शिरस्) of every book on any branch of knowledge (बिद्या) which is a paronym of and derived from the same root (बिद् Video) with Veda (बेद्). The Tantra calls it as the heading of the Gáyatrí which begins with the syllable;
तसेतन्महा गुह्यं गायत्र्याः शीर्षमुच्यते ।
ॐ भूर्भुबः स्वरित्यादिः । गायत्रीतन्त्रं ॥
2. The sacred Syllable. पूर्णाक्षरं It is called the sacred syllable because it is used in sacred writings and in the sacred Vedic and Sanskrit languages only, and never in the popular vernacular tongues, which are known as unsacred and impure (असंस्कृतं). Moreover it is used in sacerdotal functions of the sacerdotal class (संस्कारेसंस्कृतात्मनां) or regenerate classes of men, and never by the impure Súdra on pain of damnation (ॐकारोच्चारणाद्धोमात् श्रुद्रोहिनरकंब्रजेत्), unless he is sacrified by investiture of the sacred or sacrificial thread, (यज्ञोपवीतं). Thus says the Sástra; श्रुद्रोऽपि अधीकारीस्यात् यज्ञसूत्रप्रदानतः । गायत्री तन्त्रं, ४र्थ पटलः ।
The sacredness of the word Om, as the expression for the eternal position of things, is specially emphasised in the Katha Upanishad (Weber. p. 158).