Therefore, after uttering "Om," the acts of sacrifice, giving and austerity, as laid down in the scriptural injunctions, are always begun by those who speak of Brahman.
Explanation
Krishna is explaining the sacred formula "Om Tat Sat" as a designation of Brahman (the ultimate reality). Here he says "Om" is the syllable that opens sacred acts: those who expound Brahman (brahmavādin) begin sacrifice (yajña), giving (dāna) and austerity (tapas) with it, following scriptural rules. It marks the action as dedicated and consecrated.