Since oṁkāra is the basic principle of all Vedic knowledge, it is uttered before one begins to chant any Vedic hymn. Without oṁkāra, no Vedic mantra is successful. The Gosvāmīs therefore declare that praṇava (oṁkāra) is the complete representation of the Supreme Personality of Godhead, and they have analyzed oṁkāra in terms of its alphabetical constituents as follows:
Oṁkāra is a combination of the letters a, u and m. A-kāreṇocyate kṛṣṇaḥ: the letter a (a-kāra) refers to Kṛṣṇa, who is sarva-lokaika-nāyakaḥ, the master of all living entities and planets, material and spiritual. Nāyaka means “leader.” He is the supreme leader (nityo nityānāṁ cetanaś cetanānām). The letter u (u-kāra) indicates Śrīmatī Rādhārāṇī, the pleasure potency of Kṛṣṇa, and m (ma-kāra) indicates the living entities (jīvas). Thus oṁ is the complete combination of Kṛṣṇa, His potency and His eternal servitors. In other words, oṁkāra represents Kṛṣṇa, His name, fame, pastimes, entourage, expansions, devotees, potencies and everything else pertaining to Him.
Ref >> Srila Prabhupada Vani.