The Lord has innumerable incarnations, and omkara is one of them. As Kṛṣṇa states in Bhagavad-gita: “Amongst vibrations, I am the syllable om”. (Bg. 9.17) This means that omkara is non-different from Kṛṣṇa. Impersonalists; however, give more importance to omkara than to the Personality of Godhead, Kṛṣṇa. The fact is; however, that any representational incarnation of the Supreme Lord is non-different from Him. Such an incarnation or representation is as good spiritually as the Supreme Lord. Omkara is therefore the ultimate representation of all the Vedas. Indeed, the Vedic mantras or hymns have transcendental value because they are prefixed by the syllable om. The Vaisnavas interpret omkara as follows: by letter O, Kṛṣṇa, the Supreme Personality of Godhead, is indicated; by the letter U, Kṛṣṇa’s eternal consort Srimati Radharani is indicated; and by letter M, the eternal servitor of the Supreme Lord, the living entity, is indicated. Omkara is as good as any other incarnation of the Supreme Lord and is the most auspicious representation of the Supreme Lord.
In Brahma-samhita a nice description of the flute of Kṛṣṇa is given: “When Kṛṣṇa began to play on His flute, the sound vibration entered into ear of Brahma as the Vedic mantra om”. This om is composed of three letters— A, U, and M – and it describes our relationship with the Supreme Lord, our activities by which we can achieve the highest perfection of love and the actual position of love on the spiritual platform. When the sound vibration of Kṛṣṇa’s flute is expressed through the mouth of Brahma, it becomes gayatri. Thus by being influenced by the sound vibration of Kṛṣṇa’s flute, Brahma, the supreme creature and first living entity of this material world, was initiated as a brahmana.
Source : A.C. Bhaktivedanta Swami Prabhupada (2012 edition), “Teachings of Lord Caitanya, The Golden Avatara”, Page 254, 255, and 361