tasmād omityudāhṛtya yajñadānatapaḥkriyāḥ | pravartante vidhānoktāḥ satataṁ brahmavādinām ||17.24||
Therefore, for those who know the Vedas, the activities such as rituals, charities, and religious disciplines, mentioned by injunctions (of the Veda), always commence after uttering “Om”.
All these activities prescribed by Vedas should start by reciting Om to ensure their efficacy. By reciting Om makes all these things as effective as if they were done exactly as mentioned in the mandates of śāstra - vidhānoktāḥ. In Taittirīyopanisad stated, “Om iti brahma - Om is brahman”. In the context of upāsana upon Om is the verbal / sound form which denotes brahman. It is a support / means for meditation upon para-brahman / nirguna-brahman - attribute-less brahman as well as apara-brahman / saguna-brahman - attributed brahman. “The sound Om should be meditated upon as brahman, because Om is everything.” Since everything in the form of words pervades by Om, where the content of Om is a,u,m represents all words which are all names and forms, therefore it is the most exalted word to represent brahman. It is also mentioned here when to use the word Om.
In the common context “Om is the expression of approval that is well known.” In the ritual context “chant Om, make the devatā listen. Singing sāmāveda after uttering Om. Having chanted Om śom, they chant rkveda. Yajurveda priests chant mantra of appreciation having uttered Om. The yajamana permits the performs of agnihotra having uttered Om.” And in the context of vedānta studying, “When students chant Om, with the idea “may I attain brahman”, he definitely attains brahman.”
Next Lord Krsna is talking about tat.
tadityanabhisandhāya phalaṁ yajñatapaḥkriyāḥ |
dānakriyāśca vividhāḥ kriyante mokṣakāṅkṣibhiḥ ||17.25||
Saying “tat,” various activities (such as) rituals and religious disciplines and charities are performed by those who want mokṣa, without expecting a result (other than antaḥ-karaṇa-śuddhi).
The word “tat” is used by those who want mokṣa - mokṣakāṅkṣibhiḥ, when they perform various activities such as rituals, religious disciplines, or charitable activities - yajñatapaḥkriyāḥ. These actions are performed without expecting a result such as punya etc - anabhisandhāya phalaṁ.
The word tat indicates brahman, as it does in the sentence, “tat tvam asi.” (Chāndogyopanisad). In this upanisad, the mahavakya “you are that (brahman)” comes nine times, and the first time occurs in 8th chapter 6.8.7. Logic is given for us to understand the fact that I am sat brahman alone. Jīva is nothing but brahman with upādhi in the form of mind, therefore in the deep sleep state, when mind is suspended, jīva comes back to its own nature ānanda. It is illustrated that the existence sat has entered in the mind as the very self in the form of jīvātma, like a person enters into a mirror in the form of reflection or like the sun enters in the water (different water body) in the form of reflection. Being identify and obtaining in the mind then alone jīva is capable for transactions like seeing, hearing, etc. When there is resolution of that mind, then jīva is back into his own nature which is sat. In the deep sleep, jīva is resolved into his true self, just like by removal of the mirror, the reflection of a person which is obtained in the mirror becomes merged into the person as though. Similarly when there is resolution of the upādhi, brahman which entered into the mind in the form of reflected consciousness, comes back to its true self.
That paramātma is said to attain his own true nature, by suspending the status of being jīva which is called mind and giving up the connection between sense organs and objects. Therefore in the deep sleep state ānanda alone is there (with ignorance). This is how Upanisad presented tat brahman which is the nature of you - tvam. This nature of ānanda is owned up in the waking state by understanding that my association with this upadhi is not real, since upadhi doesn’t have independent reality on its own, then the association with it is also mithyā. Since the association is mithyā, satyam knowledge alone is the solution. Therefore the teacher tells his student, “tat tvam asi - you are that”.