The Chhandogya Upanishad (Chandogya Upanishad) is considered to be one of the most ancient, if not actually the oldest, of the major Upanishads. In its sprawling scope it includes a number of different teachings, vidya, as it seeks to illuminate the ‘secret of the veda’ for a developing humanity.
Sri Aurobindo provides an historical context: “After the destruction of the conservative Kurus and Panchalas at Kurukshetra, the development of the Vedanta commenced and went on progressing till in its turn it reached its extreme and excessive development in the teachings of Buddha and Shankaracharya. But at the period of the Chhandogya it is in its early stage of development. The first sections of the Upanishad are taken up with an esoteric development of the inner meaning of certain parts of the sacrificial formulae, which in itself is sufficient to show that the work belongs to the first stratum of Vedantic formation..”
As with all the Upanishads, the Chhandogya is concerned with the ultimate knowledge of “that which, being known, all is known”. The knowledge of Brahman. It approaches this knowledge from a number of different directions. Sri Aurobindo has focused on brief sections of this extensive Upanishad, as he has translated specific segments of Chapter 1, and provided brief commentary on this opening section, as well as on one of the latter segments of the text. The first section is focused on OM, the udgitha, in a very similar manner to that seen in the Brihadaranyaka Upanishad. The importance of OM and its esoteric significance can be seen from the multiple treatments provided in various Upanishads.
Sri Aurobindo, The Upanishads, Chhandogya Upanishad, pp.349-366