tasmād oḿ ity udāhṛtya
pravartante vidhānoktāḥ
satataḿ brahma-vādinām

Translation of Bhagavad Gita 17.24

Therefore, transcendentalists undertaking performances of sacrifice, charity and penance in accordance with scriptural regulations begin always with om, to attain the Supreme.

Commentary by Sri A.C. Bhaktivedanta Swami Prabhupada of Gaudiya Sampradaya:

Om tad visnoh paramam padam (Rig Veda 1.22.20). The lotus feet of Vishnu are the supreme devotional platform. The performance of everything on behalf of the Supreme Personality of Godhead assures the perfection of all activity.

Commentary by Sri Vishvanatha Chakravarthi Thakur of Gaudiya Sampradaya:

(Combined commentary to texts 23 and 24)

The three types of austerity, sacrifice, charity, foods and faith  have been described generally for all human beings. However, among the sattvika people there are also the seekers of brahman. All of their sacrifices, charities, austerities and food indicate brahman. That is explained in this verse.

The three words om tat sat, indicating brahman, have been revealed by the sages (smrtah). Among those words, orh is well known in all the srutis as the name of brahman. The word tat is well known to designate the cause of the universe, and is also known to signify the obliterator of what is not tat -the material world. Sat is defined in the srutis as that which exists before everything else (eternal): sad eva saumyam agre asit. (Chandoyga Upanisad 6.2.4) Since the brahmanas, sacrifices and the Vedas were created in ancient times by brahman itself (tena) in the form of these sounds orh tat sat, the sacrifices, austerities, charity and other works of the present seekers of  brahman are started uttering (udahrtya) the name of brahman, om.

Commentary by Sri Ramanuja of Sri Sampradaya:

17.24 After pronouncing Om, the Vedas are recited. Thus the connection of the syllable Om with the Vedas and the rituals enjoined in the Vedas such as sacrifices has been described. As the Vedas are connected with the word Om, all belonging to the three stations of life who memorise the Vedas and perform the rituals inculcated in them, are included in the expression, ‘expounders of the Vedas.’ Now, He describes the manner by which the word Tat is connected with them:

Commentary by Sri Sridhara Swami of Rudra Sampradaya:

In order to convey the supreme, eternal auspiciousness of OM TAT SAT, Lord Krishna first glorifies the pranava OM. Pranava from the root word prana meaning breath, refers to the first breath and is the sound of OM beginning all Vedic rites. In the performance of all Vedic rites such as yagna or propitiation, tapah or austerities and danam or charity; the pranava OM is always reverently vibrated as the first audible sound. The transcendental sound of OM is the personalised vibration of the Supreme Lord, it is most auspicious and eternal. By its transcendental potency all activities performed by the Vaisnavas and brahmanas enjoined by the ordinances of the Vedic scriptures are auspiciously begun with OM exclusively and by doing so insures that whatever may be defective becomes efficacious.

Commentary by Sri Madhvacharya of Brahma Sampradaya:

Once again the Supreme Lord Krishna describes the modalities of actions that are essential to be performed by revealing OM TAT SAT. These three divine syllables headed by the transcendental sound vibration OM are all verily nomenclatures of the Supreme Lord. All over creation the Supreme Lord pervades by His internal potency with the omnipresence of OM. As the Supreme Lord is glorified with the greatest respect in all the Vedic scriptures, He is known as TAT. Since the Supreme Lord is perfect without any defects and completely auspicious, He is known as SAT. The following information is confirmed in the Rig Veda: OM TAT SAT is vibrated for propitiation to the Supreme Lord, adherence to the Vedic ordinances and injunctions and honouring creation, the brahman or spiritual substratum pervading all existence and the atma or immortal soul within all jivas or embodied beings.

Commentary by Sri Keshava Kashmiri of Kumara Sampradaya:

The transcendental evocation OM TAT SAT denotes the Supreme Lord Krishna through His abstract auxiliary potency the brahman or spiritual substratum pervading all existence and is all auspiciousness. Due to this interaction the recitation of OM TAT SAT by duly initiated Vaisnavas and brahmanas at the commencement of all Vedic rites insures success and is ascertained as the cause of accomplishing all Vedic rituals successfully causing well being and attainment to the Vaisnavas and brahmanas who are the topmost members of creation. Now in order to delineate the proper actions involved with each for achieving the desired goal, Lord Krishna begins with OM explaining that it is a name for the brahman and is a potency of the Supreme Lord as it is connected to the prime source of all the Vedic scriptures. Therefore by vibrating the transcendental pranava OM at the commencement of any prescribed Vedic activity without any pause or digression stabilises the continuity and connection insuring successful accomplishment of yagna or propitiation and worship by Vaisnavas and brahmanas, tapah or austerities by the ksatriyas or royal warrior class and danam or charity by the vaisyas or merchant class.

Commentary by Sri Adi Shankaracharya of Advaita Sampradaya:

17.24 Tasmat, therefore; yajna-dana-tapah, kriyah, acts of sacrifice, charity and austerity-acts in the form of sacrifice etc.; vidhana-uktah, as prescribed through injunctions, as ordained by the scriptures; brahma-vadinam, of those who study and expound the Vedas; satatam, always; pravartante, commence; udahrtya, after uttering; om iti, the syllable Om.

Commentary by Sri Abhinavagupta of Kaula Tantra Sampradaya:

17.23-27 Om etc. upto abhidhiyate. An indication of the Brahman i.e., an act of facing (or aiming at) the Brahman is made by these three words viz., OM, TAT and SAT. Of them, Om indicates that ‘This purport of the scriptures is [to be ?] accepted as long as one has bodily connection’. The pronoun TAT, which denotes generality and which is incapable of denoting exclusively a particular, mentions, as far as the Brahman is concerned, the absence of intention for fruit. An intention is not possible without reference to something particular. No doubt [TAT] may denote all particulars [in general]. But it would lead to intending the fruits and the doer-ship with regard to all [in general]. Even then [the Brahman] cannot have connection with any particular fruit. The Vedic word SAT denotes ‘prasie’. The act, like this sacrifice etc., though perormed, truns out to be an act of the Tams (evil act) if it is performed with the idea, ‘It is an evil act’. Further, what is performed with an intention for a particular fruit is not praiseworthy, and it causes nothing but bondage. Therefore, those, who bear in mind, ‘This is a thing to be performed’ – they are not fettered, even though they perform acts like sacrifice etc. With this idea only, it has been said in the Adiparvan as – ‘The austerity is not dirt, nor the [Vedic] study, nor the natural ritual enjoined by the Vedas, and nor the act of earning wealth by all efforts. But, if they are struck by mind, they themselves become dirt.’ (MB, Adi. i, 210). [Here in this passage] dirt that which fetters. Natural : [That is, ordained ], such as – ‘Without expecting anything, a Brahman should learn the Vedas etc., together with their six subsidiaries’ etc. By all effort : by the act that is suitable and is well known in the scriptures and in the worldly practice. By mind : by the mind that is yoked to the traid of the Strands, Sattva etc. [Those actions that] have been simply ruined [by that mind], are binding and not otherwise. This is the idea here. Therefore, the acts like sacrifice etc., and the acts, like acquiring [Wealth] etc., for that purpose, have to be perfored as being inevitable as long as the body exists. Or, perhaps, the word OM conveys [with regard to the Brahman] the idea of That in Which the entire universe has been totally calmed down. TAT conveys the idea of That of the nature of Sovereign Independence of Will, which is nothing but comprehending the rising waves in the form of the universe. And the word SAT denotes the act of manifestation by the Brahman – even though It is complete in Itself – as [the universe] having varied nature, a manifestation that causes differences (or duality) expanding under the weight of its Sovereign Independenc of Will. Hence it has been said ‘In the sense of manifesting as beings and in the sense of manifesting perfectly [SAT is employed].’ Thus, having is front [as a casue] that aspect (the Soul) of highly tranquil nature; and having in the centre that aspect which is well connected with desire-waves viz., the desire to make gift, the desire to perform sacrifice and the desire to observe penance; this final body radiates (or dances) filled with a group of activities, such as offering gift, performing sacrifice and observing austerity, and of what are conducive to them. This triad is indeed simultaneously the inherent nature of the Unhindered One (the Soul). So, what fruit could be there ? And to whom ? How ? Whence ? Where ? And by what means ? The act of person, with no faith, is an act of the Tamas and bears no fruit in any way and bears fruit which is nothing but the labour undertaken in bringing together the band of means [of those acts]. Hence one should not on any account remain faithless. This is being said now :

Sanskrit Shloka Without Transliteration Marks:

tasmad om ity udahrtya
pravartante vidhanoktah
satatam brahma-vadinam

Sanskrit to English Word for Word Meanings:

tasmat — therefore; o? — beginning with o?; iti — thus; udah?tya — indicating; yajña — of sacri?ce; dana — charity; tapa? — and penance; kriya? — performances; pravartante — begin; vidhana-ukta? — according to scriptural regulation; satatam — always; brahma-vadinam — of the transcendentalists.