Verse 1.5.3

अथाध्यात्मं य एवायं मुख्यः प्राणस्तमुद्गीथमुपासीतोमिति ह्येष स्वरन्नेति ॥ १.५.३ ॥

athādhyātmaṃ ya evāyaṃ mukhyaḥ prāṇastamudgīthamupāsītomiti hyeṣa svaranneti || 1.5.3 ||

3. Next, this is how worship concerning the physical body is performed: One should worship the chief prāṇa as udgītha, for it seems to say Om as it makes the organs [of perception and action] function.

Word-for-word explanation:

Atha, now; adhyātmam, concerning one’s own body; yaḥ eva ayam mukhyaḥ prāṇaḥ, that which is the chief prāṇa [which divides itself into five functions: prāṇa, apāna, vyāna, udāna, and samāna]; tam udgītham upāsīta, one should worship as udgītha; hi, for; eṣaḥ, this [prāṇa]; om iti svaran, uttering Om; eti, activates [all the organs—of speech, vision, etc.].

Commentary:

As far as the physical body is concerned, the Upaniṣad says to treat the chief prāna as Om. The chief prāṇa is responsible for what the organs of perception (the eyes, ears, nose, etc.) and the organs of action (the hands, feet, speech, etc.) do. It makes them active. Just as some people say Om before they do or say anything, so prāṇa seems to say Om as it makes the organs function. Not that it really says Om, but because the organs cannot act without its support, it is suggested that the chief prāṇa has to give the signal (that is, by saying Om) and then only do the organs start operating.