Prashna-5-5-yaḥ punaretaṃ

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

But he who meditates upon the supreme Purusha within the sun, by this pratîka, i.e., substitute, i.e., by the syllable ‘Om’ with the knowledge that it is of three mâtras, by such meditation becomes united with the sun. According to the context, the syllable ‘Om’ must be taken to be a help, being a Pratîka or substitute, from the declaration of its identity with the Brahman, higher and lower, according to the sruti. Otherwise, the accusative case of ‘Om’ used in many places, will be objectionable. Though by the use of the third case, the syllable ‘Om’ may be understood as a kârana, i.e., instrument, still agreeably to the context, it must be read as if in the accusative case, the meaning then being, ‘let one meditate upon the syllable of three mâtras as the supreme Purusha.’ According to the maxim ‘you may abandon one for the benefit of a whole family’ the instrumental case should be here given up for the accusative case used in previous passages. By such meditation, he becomes united with the bright sun. Then, even if he dies while meditating, he does not return from the solar world as from the lunar, but is for ever united with the sun. Just as the snake puts off its skin and becomes new again, its skin having been peeled off, so—as in this illustration—this man being freed from the impurity of sin, analogous to the skin, is conducted up, by the sâmans representing the third mâtra, to the world of Brahma, i.e., Hiranyagarbha called Satya. He, Hiranyagarbha is the âtman of all the jîvas travelling in samsâra; for, he is the internal âtman of all living beings in the subtle form; and in him the subtle âtman are all the jîvas strung together. So he is jîvaghana (dense with lives). The knower of the syllable ‘Om’ of the three mâtras sees the Purusha called Paramâtman beyond this Hiranyagarbha and sees him by meditation as lodged in all bodies. The following two verses make the drift stated clear.

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