Prashna-5-2-etadvai satyakāma

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

This Brahman—the higher Brahman, true, undecaying, known as Purusha, and the lower Brahman known as Prâna, the first-born—is, indeed, the syllable ‘Om’; for, the syllable ‘Om’ is its Pratika, substitute. The higher Brahman, not capable of being indicated by words, etc., and devoid of all distinguishing attributes, is, therefore, being beyond the reach of the senses, incapable of being comprehended by the mere mind. But to those worshippers who contemplate on the syllable ‘Om’ as upon the image of Vishnu, etc., and regard it as a substitute for Brahman, the Para Brahman is understood to reveal itself, from the authority of the sâstrâs; so too the lower Brahman. Therefore, it is by courtesy, that Brahman, the higher and the lower, is said to be the syllable ‘Om.’ Therefore, he who knows thus, attains either the higher or the lower Brahman, by this very means to the attainment of the âtman, i.e., by the meditation on ‘Om.’ For the syllable ‘Om’ is the nearest stay of Brahman.

Links
Various resources on the Prashnopanishad
Upanishad Series – Kena Upanishad Commentary by Swami Chinmayananda