Is reciting Vedic Mantras without being preceded by “Om” condemned somewhere in Vedas or in other scriptures?
The Upanishads say that the Vedas are Om. Om is the Vedas.
Why prayers and rites are started with Om is explained in Taittiriya Upanishad I.viii.1. (Swami Gambhirananda translator) - "...A Brahman, when about to recite the Vedas utters Om under the idea, 'I shall attain Brahman'. He does verily attain Brahman." At the end of his commentary on this verse, Sankara says: "The purport of the passage is that, since the activities that are undertaken with the utterance of Om become fruitful, Om should be meditated on as Brahman."
Also see Brahma Sutras 3.3.63-64 and Chandogya I.1.9.The Chandogya verse says (Swami Gambhirananda translator):
These Vedic rites are begun with it [Om]. After uttering Om (the Adhvaryu) chants the hymns (for the gods). (the Hota) praises (the gods). (the Udgata) sings. (These rites are meant) for the worship of this very Om. Through the greatness and essence of Om (are begun the rites enjoined by the Vedas).
So scripture says rites are always started with Om as they become fruitful with the recitation of the Om.
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