Taittirīyopaniṣat 01.19

ॐ इति ब्रह्म। ॐ इतीदं सर्वम्। ओमित्येतदनुकृति ह स्म वा अप्यो श्रावयेत्या श्रावयन्ति। ओमिति सामानि गायन्ति। ओगंशोमिति शस्त्राणि शगंसन्ति। ओमित्यध्वर्युः प्रतिगरं प्रतिगृणाति। ओमिति ब्रह्मा प्रसौति। ओमित्यग्नि होत्रमनुजानाति। ओमिति ब्राह्मणः प्रवक्ष्यन्नाह ब्रह्मोपाप्नवानीति। ब्रह्मैवोपाप्नोति ॥१९॥

"Om" is Brahman. "Om" is all this. With "Om" as this imitation, indeed, verily, the priests make (someone) hear, thus they make (someone) hear. With "Om" they sing the Sāman chants. With "Om" and "Sham" they recite the hymns of praise. With "Om" the Adhvaryu priest accepts the response. With "Om" the Brahmana priest initiates. With "Om" Agni permits the oblation. With "Om" the knower of Brahman, about to declare, says, "I have attained Brahman." He indeed attains Brahman.

Taittirīya Bhāshya 01.19

Testimonial from Pancharatra is quoted paraphrasing the same, attributing "Om" to Lord Vishnu because of his superior qualities.

"ॐनामा भगवान् विष्णुः अधिकोच्चगुणत्वतः। यद्यद्रूपं भगवतस्तदिदं सर्वमेव च॥

"The Lord Viṣṇu, known as Oṃ, is supreme due to the superiority of qualities. Whatever forms belong to the Lord, all this indeed is that.

ओमेवाधिकपूर्णत्वात् तस्माद्यज्ञेषु चर्त्विजः। ओमित्येव स्वकर्माणि कुर्वन्त्युद्दिश्य केशवम्॥

Because Om is supremely complete, therefore, in sacrifices, the priests perform their own actions, addressing Lord Keśava, with "Om" alone.

अधिकोच्च शृणुष्वेति हरिमध्वर्युराह हि। अधिकोच्च महाधाम सूच्च देेवेति चाऽदरात्॥

The priest (adhvaryu) respectfully addressed Lord Hari in a louder voice, saying, 'Listen!' Again, in a louder voice, he said, 'O great abode, proclaim, O god!'

प्रति प्रतिगृणात्येनं शंरूपात्युच्च शावन। इत्याह होता शस्त्रेषु तथैवान्येऽपि चर्त्विजः॥

The hotṛ priest, as well as the other officiating priests, receive him with high praise from the auspicious form, saying so in the hymns.

एवं स्वध्यायकृच्चाह ब्रह्मप्राप्त्यर्थमञ्जसा। एवं जानन्त एते तु प्राप्नुवन्त्येव तत्परम्॥"

Thus, one who has performed self-study and declares it for the direct attainment of Brahman—such knowers indeed attain that Supreme."

इति च॥

- stated thus as well (in Tattvasaṁhitā).