The Primordial Syllable
Oṃ (also written AUM) is the praṇava — the primordial sound from which all scripture is said to emanate. In the Vedic tradition, all mantras begin with Oṃ as a mark of their sacred authority and as an invocation of the supreme. In the Tirumantram ('Oṃ Namo Nārāyaṇāya'), Oṃ heads the eight-syllable mantra as its first element.
Śrī Vaiṣṇava Interpretation
Piḷḷai Lokācārya in Mumukṣuppadi analyzes the akāra (the 'a' sound within Oṃ) as affirming Bhagavān as the first cause and inner self of all. The praṇava as a whole declares the fundamental identity of the jīva with Brahman — not in the Advaita sense of non-difference, but in the Viśiṣṭādvaita sense of the jīva as Brahman's inseparable mode (viśeṣaṇa) and servant (śeṣa).
AUM: The Three Letters
The syllable Oṃ is traditionally analyzed as comprising A + U + M: akāra representing Bhagavān, ukāra representing Śrī Lakṣmī (the inseparable consort), and makāra representing the jīva. Together they encode the entire śeṣa-śeṣī relationship in a single syllable.