Towards the end of the Seventeenth Chapter we are given the cryptic message of “ Om-Tat-Sat”, a term with which we are all familiar, but the meaning of which is not always so clear. It is said that this is a very holy expression and it has to be employed in every religious performance. We conclude all pious acts with the utterance, Om-Tat-Sat, which appears to be an invocation of God at the end of a performance. The meaning of these words is not clear, and no commentary on the Gita will perhaps be an aid to us in understanding what these three terms actually signify. We merely say, Om-Tat-Sat. We do not know what it means. Well, we may go a little deep into its significance from the point of view of the Bhagavadgita itself, in the light of the great message that has been given to us through its various Chapters. And in this light if we look at these terms, it would appear that the three seeds, Om, Tat, Sat signify the total comprehensiveness of the nature of Brahman ranging beyond the concepts of Reality in the form of transcendence and immanence. Generally, a remote thing is referred to as Tat, in the Sanskrit language. ‘That’ is Tat. We refer to God as Tat, It, etc. as a super transcendent inaccessible something. Sat is the very same transcendent Reality is hidden and present as the Divine immanence in all things. God is transcendent and also immanent. He is above us; he is inside. Now, these ideas of transcendence and immanence, - Tat-Sat,- the notions of God being outside as well as inside, are also to be transcended in a larger grasp, which is Om. Here, in this mystical significance of the well-known symbol of Om, we are given a further transcendence of both the transcendent aspect and the immanent aspect of the Absolute. It is, in the language of the Upanishad, the Bhuma, or the Plenum, the completeness whereby we cannot look upon it either as something above us or as something within us. To that supreme completeness, there are no outward and inward differences. There is no such thing as going above and being within, because it is everywhere, at all times, without the limitations of space, time and objectivity. Such an incomprehensible significance is embedded in this mystical formula of Om. Naturally, it is a holy expression, which is unutterable, beyond understanding but signifying everything that is blessed and supreme. Such is Om, which grasps within itself all that is real everywhere, the transcendent and the immanent. So, God is all, the Absolute is everything. The invocation of this Symbol in our experience, in our own consciousness, a remembrance of it at the sacred conclusion of any kind of performance, religious or otherwise, is regarded as a completion of that performance. God completes everything, and everything is incomplete where God is absent. The only thing that is full is God. And so He has to be invoked always. ~~ “ The Philosophy of the Bhagavadgita by Rev. Swami Krishnananda.”

OM
“By pronouncing OM properly, the heart and the mind are filled with peace, tranquility and bliss. You can feel the limitless ocean of power and energy within you. In order to acquire and possess this invaluable treasure of Nature, the Bliss and Peace, OM works as a key to unlock the coffer. The sound of OM, filling the entire atmosphere, gives immediate relief to humanity. At the end of the prayers in Jewish, English or Arabian languages, the word Amen, or Aamin, is nothing but the changed form of OM. The last word of the Greek language is Omegh which has the prominence of OM. Also, in the English words, omnipresent, omnipotent and omniscient, the eminence of OM is distinct. The presence of OM in these words is not just by chance. It is on account of the word Om that these words give the indication of universalism. Om is like a huge tree, casting its cool shadow all around and providing peaceful and refreshing relief and energy to all those suffering from the scorching heat of pain, affliction, worry, anxiety, etc.
In every beginning of Koran, at the top, there are three letters, Alif (A ) Lam ( l ) and Mim (m ). Lam (L ) can be pronounced as Wao ( O ), according to Arabic grammar, under certain circumstances. In Arabic Shamsaldin is read as Samsuddin. Nizamaldin is read as Nizamuddin. According to Arabic grammar, when Lam (L ) comes between a vowel and a consonant, it becomes silent and gives the sound of Arabic Pesh (O or U ). Similar is the case here with Alif (A) , Lam (L ) and Mim (M). Here Lam (L ) is in between the vowel Alif (A ) and the consonant Mim (M) and, therefore, it becomes silent and gives the sound of O or U. Accordingly, Alif, Lam and Mim give the sound of Alif(A) Wao (O or U) and Mim(M) i.e. AOM oa AUM. This is nothing but Om. This is no secret of God, as it is said.
The conch shell, when blown, gives out the sound of Om which diverges in the space to spread peace, concentration and self-confidence in the atmosphere. This stimulates the heart to be merged in Godhood. It has been since birth. When a child is born, he cries with the sound of Om. When the children play and their minds are saturated with happiness, they produce a noise which resembles that of Om. When people fall sick or are afflicted with some unbearable pain, they invariably groan which produces the sound resembling Om. This gives them relief from their agonizing pain.” ~ Excerpt from the work of Rev. Swami Ramtirth.

“The vowel sounds produced by the voice are due to the vibrations of two cartilaginous plates, the "vocal chords" placed at the top of wind pipe, edge to edge with a narrow slit between them; air blown through this slit from the lungs keeps the plate vibrating. The apparatus is really a free reed, the vocal chords have muscles attached to them which can vary the frequency of the vibration and the pitch of the sound produced. The vowel are continuous sounds formed by varying the size and shape of mouth cavity. The consonants are particular interruptions of these sounds. They cut short the vowel sound but cannot themselves be sounded without vowels. And for this reason, the vowels are known as 'Shaktis'.” ~ Excerpt from Sir John Woodroffe

AUM contains three letters; A starts from the initial manifestation with throat, U comes to the mouth and M ceases with closing of lips. These cover the entire vocal phenomenon.....all words, languages are enshrined within these three...A,U,M.