अथ खलु य उद्गीथः स प्रणवो यः प्रणवः स उद्गीथ इत्यसौ वा आदित्य उद्गीथ एष प्रणव ओमिति ह्येष स्वरन्नेति ॥ १.५.१ ॥
In plain words
Now the Udgītha is the Praṇava, and the Praṇava is the Udgītha; and that sun yonder is the Udgītha, and it is the Praṇava, for it moves sounding forth Om.
The commentators
Chāndogyopaniṣadbhāṣyam
प्राणादित्यदृष्टिविशिष्टस्योद्गीथस्योपासनमुक्तमेवानूद्य प्रण वोद्गीथयोरेकत्वं कृत्वा तस्मिन्प्राणरश्मिभेदगुणविशिष्टदृष्ट्या अक्षरस्योपासनमनेकपुत्रफलमिदानीं वक्तव्यमित्यारभ्यते—
अथ खलु य उद्गीथ स प्रणव बह्वृचानाम्, यश्च प्रणव तेषा स एव च्छान्दोग्ये उद्गीथशब्दवाच्य । असौ वा आदित्य उद्गीथ एष प्रणव , प्रणवशब्दवाच्योऽपि स एव बह्वृचानाम्, नान्य । उद्गीथ आदित्य कथम् ? उद्गी थाख्यमक्षरम् ओमिति एतत् एष हि यस्मात् स्वरन् उच्चा रयन्, अनेकार्थत्वाद्धात्नाम्, अथवा स्वरन् गच्छन् एति । अत असावुद्गीथ सविता ॥
In plain words
Having reviewed the meditation on the Udgītha as associated with prāṇa and the sun, and having established the oneness of the Praṇava and the Udgītha, the text now begins to explain the meditation on the imperishable syllable, endowed with the distinct attributes of prāṇa's rays, which grants the reward of many sons.
Now, whatever the udgītha is, that is the praṇava for the chanters of the Ṛgveda; and whatever the praṇava is, that same is named udgītha in the Chāndogya. That very sun is the udgītha, and this is the praṇava. For the chanters of the Ṛgveda, the praṇava is indeed that alone, and nothing else. How is the sun the udgītha? The syllable named udgītha is oṃ; for as one chants and utters it, because verbal roots have many meanings, or because it moves as it resounds, therefore that sun is the udgītha, the sun.
Close to the Sanskrit
Having restated the meditation on the udgītha qualified by the vision of prāṇa and āditya, and having established the identity of praṇava and udgītha, now it is commenced to be explained the meditation on the imperishable syllable, characterized by the vision of the differentiation of prāṇa's rays, which yields the fruit of many sons.
Now indeed, whatever is the udgītha that is the praṇava of the Ṛgveda chanters, and whatever is the praṇava, that same is designated by the word udgītha in the Chāndogya. That sun indeed is the udgītha, this praṇava; and the praṇava is designated also as that of the Ṛgveda chanters, not another. How is the sun the udgītha? The syllable named udgītha is Oṃ, this one, for chanting and pronouncing it, because of the multivalent nature of roots, or else going while sounding. Hence that sun is the udgītha.