The aṣṭākṣara ('eight-syllabled') mantra — Oṃ Na-mo Nā-rā-ya-ṇā-ya — is the Tirumantram, the most sacred mantra of the Śrī Vaiṣṇava tradition. It is one of the three rahasyams (sacred mysteries) revealed to a disciple at the time of pañca-saṃskāra initiation by the Āchārya. Piḷḷai Lōkācāryar's Mumukṣuppadi ('The Steps for the Seeker of Liberation') is the definitive Śrī Vaiṣṇava commentary on the aṣṭākṣara, revealing layer upon layer of meaning in its three component parts: the praṇava, the namaḥ, and the Nārāyaṇāya.
The praṇava ('Oṃ') is the vācaka (the denoting syllable) of Nārāyaṇa. Its three constituent phonemes — a, u, and ma — are expounded to reveal: (a) a = Viṣṇu as the supreme independent reality (Svātantra Brahman); (u) = the ātmā's exclusive belonging to Viṣṇu alone (ananya-śeṣatva); (ma) = the ātmā itself, understood as atomic consciousness that finds its fullness only in the Lord. Together, the praṇava encodes the ātmā's svarūpa: it is wholly dependent upon (paratantra), wholly belonging to (śeṣa), and wholly inseparable from Bhagavān. The namaḥ (namo in sandhi) is the ātmā's definitive renunciation of ahaṅkāra (the false identification of the self with the body) and mamakāra (the sense of 'mine-ness' that claims the ātmā, its actions, and their fruits as its own). The namaḥ in the Tirumantram is thus not a passive salutation but an active, ontological declaration: the ātmā asserts that it does not belong to itself (sva), does not act for itself, and does not enjoy fruits for itself — all these belong to Bhagavān alone. The Āchāryas note that the full renunciation implicit in namaḥ makes it, paradoxically, the most liberating syllable in the entire mantra.
The Nārāyaṇāya is the name of Bhagavān in the dative case — 'to/for Nārāyaṇa.' The dative ending (āya) indicates that Nārāyaṇa is the recipient of everything the ātmā does, the goal toward which all kainkaryam (service) flows, and thus both the upāya (means) and upēya (end) for the surrendered soul. The name Nārāyaṇa itself is exegeted: nāra = all souls and matter; ayaṇa = the one in whom all souls dwell and who dwells in all souls — conveying simultaneously Bhagavān's immanence (antaryāmin), transcendence (Para-svarūpa), and the relationship of śeṣī (Lord) to śeṣa (servant). The aṣṭākṣara is thus a complete philosophy in eight syllables: the nature of the ātmā, the nature of Bhagavān, the obstacle (mamakāra/ahaṅkāra), and the goal — all encoded in its metre, its grammar, and its sound.