Oṁkāra is the representation of the Supreme Personality of Godhead in sound. This form of His holy name is accepted as the transcendental vibration (mahā-vākya), by virtue of which the temporary material manifestation has come into being. If one takes shelter of the sound representation of the Supreme Personality of Godhead (oṁkāra), he can realize his constitutional identity and engage in devotional service even though in conditional life.
Tat tvam asi is accepted as the primary vibration by one who does not accept praṇava, the transcendental sound incarnation of the holy name of the Lord, as the chief principle in Vedic literature. Tat tvam asi is a warning to the living entity not to mistake the body for the self. Therefore tat tvam asi is especially meant for the conditioned soul. The chanting of oṁkāra or the Hare Kṛṣṇa mantra is meant for the liberated soul. Śrīla Rūpa Gosvāmī has said, ayi mukta-kulair upāsyamānam (Nāmāṣṭaka 1). Thus the holy name of the Lord is chanted by the liberated souls. Similarly, Parīkṣit Mahārāja says, nivṛtta-tarṣair upagīyamānāt (Bhāg. 10.1.4). The holy name of the Lord can be chanted by those who have fully satisfied their material desires or who are fully situated on the transcendental platform and devoid of material desire. The name of the Lord can be chanted by one who is completely freed from material contamination (anyābhilāṣitā-śūnyaṁ jñāna-karmādy-anāvṛtam).
Ref >> Srila Prabhupada Vani.