What is OM TAT SAT?
Excerpted from the enlightening elucidation of Srimad Bhagavad Gita by revered master Anandmurti Gurumaa, this article sheds light and unravels the essential meaning of the appellation OM TAT SAT quoting the select verses.
otat sad iti nirdeśo brahman as tri-vidhah smr tah
brāhman ās tena vedāś ca yajñāś ca vihitāh purā
OM TAT SAT is the three-fold appellation of the Satchidānanda Brahman. And it is that very absolute reality which created the  Brāhmanās, Vedās and Yagyās in primeval, ancient times.
Sri Krishna says that the three-fold appellation of the Brahman viz. OM TAT SAT is the source of the entire creation with the Brāhmanās i.e. the rishis and the Vedās being created in the very beginning. Veda means gyāna, higher knowledge. Gyāna existed even when the four Vedās had not yet come into existence. Gyāna precedes even the Vedās. Thus it is gyāna that emerged from Satchidānanda Brahman. And then the sages who incorporated the gyāna in their antahkaran, talked about it through the Vedās. So know that Brahmin or Brāhmana refers to the one who has lifted the veil of ignorance, the veil of Maya, who has realised the Brahman, the absolute truth.
tasmād oity udāhr tya yajña-dāna-tapah -kriyāh
pravartante vidhānoktāh satatabrahma-vādinām

Therefore, students and followers of Vedic scriptures commence acts of yagyās, charity and austerity with utterance of OM. OM is the root of everything including the Vedās and other scriptures. That is why one should commence any yagyā, charity or austerity with the utterance of OM. The entire world and the Brahman are held in the three syllabled OM. The entire Prakriti is in its three syllables (A, U, M) whereas the amātrā (non-syllable) is the infinite substratum, the absolute Brahman. Thus utterance of OM is veritably beneficial.

tad ity anabhisandhāya phalayajña-tapah -kriyāh
dāna-kriyāś ca vividhāh kriyante moks a-kā ks ibhih
Utterance of TAT is done by the seekers of liberation who regard everything as belonging to the Lord and thus perform all actions like yagyās, austerity and charity without seeking any fruit. Everything belongs to the Lord and I too belong to the Lord, and thus I perform yagyās, austerities and charity only for the joy of doing. I don‘t desire anything in return. I seek no fruit of the actions. And this is the only way I know to live, this is my nature, I see it as my duty and I very much enjoy doing so. Now, this is the attitude of a superior, a noble human being. Such a person doesn‘t need any auspicious day to wake up early. He wakes up early morning every day and engages in his spiritual practices, for he derives joy from doing so. Some people don‘t get up even when someone tries to wake them, whereas some wake up after being cajoled, and then there are those who wake up on their own. These three categories of people are tāmasic, rājasic and sāttvic respectively.

Likewise, the one who is not charitable or who gives only because of his attitude of one-upmanship is tāmasic. For example, a fellow refuses to donate to a charity. But when his wife informs him that his neighbour has given such and such amount, the fellow‘s ego is stirred up and he says, well, in that case I will give twice as much, for who does he think he is?! Now, this is tāmasic nature. Some, on the other hand, give because they think by doing so their desires will get fulfilled, they will get something back in return. These folk are of rājasic disposition. Then again, there are a few who give without any expectations, without any desires, who derive joy from the act of giving, who regard charity as sharing the Lord‘s wealth with the Lord‘s beings, and who serve the Guru with the same devout faith – they are of sāttvic disposition. Better is rājasic than tāmasic and best of all is sāttvic. Thus see where you stand and endeavour to better yourself. So strive to give at least sometimes without any desire or expectation, as well as strive to serve sages and Gurus at least sometimes without seeking anything in return.

When service becomes a joy, when serving or giving becomes a part of one‘s nature, when waking up early and performing one‘s spiritual practices becomes a matter of course – these attributes indicate sāttvic disposition. And Sri Krishna says such sāttvic folk easily attain higher realms of dhyāna and gyāna.
sad-bhāve sādhu-bhāve ca sad ity etat prayujyate
praśaste karman i tathā sac-chabdah pārtha yujyate
O Arjuna, SAT, this name of God, represents the ultimate reality and also a generic sense of goodness and uprightness. And the word SAT is also used in context of commendable, meritorious acts.

That which exists in all times, that which is untouched by time, that which transcends the realm of time, that which is the substratum for the existence of time, that in which time dissolves – this eternal, timeless entity is known as SAT (truth, ultimate reality). Thus this word should be used judiciously, for SAT is equivalent to God, a synonym for Brahman. So if you want to lie, then at least don’t swear on the Guru’s name or God’s name; don’t blatantly speak untruth in the name of the Guru or God!

Steadfastness in yagya, dāna and tapa is also regarded as SAT. Furthermore, action offered unto the Lord is also firmly held as SAT. SAT is the backbone, the underpinning substratum, not only of your body, your senses, your mind and your intellect but also of this entire manifest world. You are not separate from SAT and you cannot be separated from SAT, for SAT is the word representing God.

SAT i.e. truth always prevails. And remember that when someone lies, even therein the substratum is still of the truth. Truth was, truth is and truth will always be. A-SAT (untruth, unreality) is always short lived – then be it a spoken lie or this whole wide world. Yes, this entire manifest world is also untruth, unreal and hence has a finite shelf life. It is ephemeral, transient. Truth alone is eternal. Truth alone is the fundamental crux of everything.