In plain words
Therefore, uttering ‘Oṃ,’ the acts of yajña, giving, and tapas prescribed by rule are always begun by the expounders of the Veda.
Word for word
Therefore, having uttered 'Om', the acts of yajña, dāna and tapas enjoined by rule always proceed, of the expounders of the Veda.
तस्मादोमित्युदाहृत्य यज्ञदानतपःक्रियाःप्रवर्तन्ते विधानोक्ताः सततं ब्रह्मवादिनाम्
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
तस्मात् ‘ओम् इति उदाहृत्य’ उच्चार्य यज्ञदानतपःक्रियाः यज्ञादिस्वरूपाः क्रियाः प्रवर्तन्ते विधानोक्ताः शास्त्रचोदिताः सततं सर्वदा ब्रह्मवादिनां ब्रह्मवदनशीलानाम् ॥ २४ ॥
In plain words
So: with Om spoken first — actually pronounced aloud — the acts of sacrifice, giving and austerity get under way. Acts of that very kind, sacrifice and the rest. Stated by the injunction, which is to say prompted by scripture. Constantly, meaning always. Among those who expound Brahman — people whose habit of speech is Brahman.
Close to the Sanskrit
Therefore, 'having uttered Om' — that is, having pronounced it — the yajña-dāna-tapaḥ acts, that is, acts having the nature of sacrifice and the rest, proceed; stated by the injunction, that is, prompted by scripture; constantly, that is, always; of the expounders of Brahman, that is, of those whose habit is the speaking of Brahman.