Om

oṁ tatsaditi nirdeśo brahmaṇastrividhaḥ smṛtaḥ | brāhmaṇāstena vedāśca yajñāśca vihitāḥ purā ||17.23||

“Oṁ tat sat,” is the three-fold expression of Brahman. By that, the brāhmaṇas, the (four) Vedas,and rituals were created in the beginning.

“Oṁ tat sat,” reveals brahman. All three words, independently and together reveal brahman. By that brahman, the brāhmaṇas, the (four) Vedas, and rituals were created in the beginning - brāhmaṇāstena vedāśca yajñāśca vihitāḥ purā. The word brāhmaṇas is denoting the varna, standing for all four categories of people mentioned in the Veda.

Now Lord Krsna is going to explain each one of these words.

In the Mandukya-upanisad, the meaning Om is explained in great detail. As the entire superimposed creation has its basis on brahman / ātmā, similarly all words have their basis in Om. “This letter Om is this everything (entire creation). All which were there in the past, all which are in the present and all which are going to be in the future are Om alone. And that which is nothing else, which is beyond three periods of time is Om alone.”

in the context of upāsana, Om represents parabrahman which is the basis for viśva (gross world / waking state), tejasa (subtle world / dream state) and prajña (causal / deep sleep state). Each of them is represented by the words a, u and m, respectively.

But Om itself is none of them, it is the silence which is the basis for three of them to be there. This silence is called turiya - the fourth because it occupies the fourth place in the order of exposition of brahman. When we see from the total level, this turiya brahman which is attribute-less, transcends and pervades the entire attributed gross, subtle and causal world. When we see from the individual level, the same consciousness ātmā me transcends and pervades three states of waking, dreaming and deep sleep.

Therefore “All which were there in the past, all which are in the present and all which are going to be in the future are Om alone.” All the vyakta - manifested, manifesting and will manifest is Om, and that which is unmanifest is also Om. “And that which is nothing else, which is beyond three periods of time is Om alone.” What else is there? There is only Om.