Om Tat Sat - a mantra from scriptures of Hinduism is an indicative mantra of Supreme God. "Om Tat Sat" per se is not any mantra.
Om (ॐ) is a straight forward mantra of
Brahm (ब्रह्म) / Kaal. Tat (तत्) is a coded mantra of Par Brahm and Sat (सत्) is a coded mantra of Purna Brahm / Supreme God.
The speaker of Bhagavad Gita explains this in detail in Gita chapter 17 verse 23 - 26Shrimad Bhagavad Gita (श्रीमद भगवद गीता) Adhyay / Chapter 17 (अध्याय १७) Verse / Shlok 23 - 26 (श्लोक २३ - २६)
Gita Chapter 17 verse 23
ॐ, तत्, सत्, इति, निर्देशः, ब्रह्मणः, त्रिविधः, स्मृतः,
ब्राह्मणाः, तेन, वेदाः, च, यज्ञाः, च, विहिताः, पुरा।।23।।
In Adhyay 17 from Shlok 23 to 28, it is stated that for the attainment of purna Parmatma (Supreme God), Om' (ॐ) - Tat' - Sat', these are the three naams.
In Gita Adhyay 8 verse 13, Brahm says that the mantra of my attainment is "Om (ॐ)"
ओम्, इति, एकाक्षरम्, ब्रह्म, व्याहरन्, माम्, अनुस्मरन्,
यः, प्रयाति, त्यजन्, देहम्, सः, याति, परमाम्, गतिम्।।13।।
The speaker of Shrimad Bhagavad Gita is saying that in that mantra of three words (Om, Tat, Sat), there is only one word Om/ॐ for me i.e. Brahm, which has to be chanted for doing sumiran. A devotee who while doing sumiran-sadhna unto last breath, gives up his body, he attains the supreme state i.e. salvation.
{Brahm has called his salvation as (anuttamam) very inferior/very bad in Gita Adhyay 7 Shlok 18.
Tat (तत्) is the mantra of Par Brahm (Kshar Purush - क्षर पुरुष)
This Tat mantra is a coded (hidden) mantra, which is jaap mantra of ParBrahm and in the jaap of Satnaam through breaths, the breath 'iti' i.e. ends on the Tat mantra.
Gita Chapter 17 verse 25
तत्, इति, अनभिसन्धाय, फलम्, यज्ञतपःक्रियाः,
दानक्रियाः, च, विविधाः, क्रियन्ते, मोक्षकाङ्क्षिभिः।।25।।
The breath 'iti' i.e. ends on the jaap of Tat (तत्) mantra of Akshar Purush i.e. ParBrahm, and is done by those men who perform various kinds of activities of yagya, austerity and charity without desiring the fruit and with the desire of attaining well-being i.e. by those who only wish to become completely free from the cycle of birth and death.
Sat (सत्) is the mantra of Purna Brahm (Supreme God / Param Akshar Brahm - परम अक्षर पुरुष)
Gita Chapter 17 verse 26
सद्भावे, साधुभावे, च, सत्, इति, एतत्, प्रयुज्यते,
प्रशस्ते, कर्मणि, तथा, सत्, शब्दः, पार्थ, युज्यते।।26।।
‘Sat’- this Saarnaam is used in true sense and in supreme sense at the end of Tat mantra with the name of this very Purna Parmatma, and Oh Paarth, Sat Shabd i.e. Saarnaam is only used in a superior act; i.e. is added at the end of the aforesaid two mantras Om and Tat.
