Ekakshara Upanishad

It is attached to the Krishna Yajurveda, and is a Samanya (general) Upanishad. The Upanishad discusses Om (Pranava) as the Ultimate Reality Brahman, equating it to the imperishable truth and sound, the source of the universe, the Uma, the Shiva, the Narayana, the Atman (soul) that resides in one’s heart. The one immortal syllable (Ekakshara), is described in the text as the Hiranyagarbha (the golden fetus, the sun, Brahma) the manifested universe, as well as the guardian of the universe. Ekakshara, literally "the one syllable", refers to Om of Hinduism. It refers to the primordial sound, the seed, the source of the empirically observed universe, representing the totality of manifested changing cosmos and the unchanging supreme reality Brahman.