Sri Aurobindo translates Mandukya Upanishad, Verses 8-12: “Now this the Self, as to the imperishable Word, is OM: and as to the letters, His parts are the letters and the letters are His parts, namely, A U M. The Waker, Vaishwanara, the Universal Male, He is A, the first letter, because of Initiality and Pervasiveness: he that knoweth Him for such pervadeth and attaineth all his desires: he become the source and first. The Dreamer, Taijasa, the Inhabitant in Luminous Mind, He is U, the second letter, because of Advance and Centrality: he that knoweth Him for such, advanceth the bounds of his knowledge and riseth above difference: nor of his seed is any born that knoweth not the Eternal. The Sleeper, Prajna, the Lord of Wisdom, He is M, the third letter, because of Measure and Finality: he that knoweth Him for such measureth with himself the Universe and becometh the departure into the Eternal. Letterless is the fourth, the Incommunicable, the end of phenomena, the Good, the One than Whom there is no other: thus is OM. He that knoweth is the Self and entereth by his self into the Self, he that knoweth, he that knoweth.”
OM represents all aspects of awareness in the manifestation as well as the transcendent pure awareness of Existence which is the root, foundation and basis of everything. The Rishi ensures that we do not give undue prominence to any particular aspect, and that we recognise that they are all part of one creation, and further that the Brahman is not limited by any particular manifestation or aspect of creation, but always includes, and yet is beyond, each aspect or their combination.
There are adherents to the primacy of any one of the statuses of awareness, as there are also adherents to the transcendent consciousness. The Upanishad makes it clear that they are all inextricably linked and essential for the completeness of understanding of Existence.
Sri Aurobindo, The Upanishads, Mandukya Upanishad, pp.319-321