In a sacrificial work o3m is substituted for the final vowel with the consonant, if any, that may follow it, in a sentence.,
The word यज्ञकर्मणि is understood here also. The word प्रणव means ओम् ॥ It is the name given to this syllable. This ओम् is substituted for the final letter (टि) of that word which stands either at the end of a Pada of a sacred hymns or at the end of a hemistich of such Rik. Thus for अ॒॒पां रेतां॑सि जिन्वति (Rig. VIII. 44. 16), we may have अपां रेतांसि जिन्वतो꣡ ३म् ॥ So also for दे॒॒वाञ् जिगा॑ति सु॒॒म्नयुः (Rig. III. 27. 1), we may have देवाञ् जिगाति सुम्नयो꣡ ३म् ॥
The word टि is repeated in this aphorism in spite of its anuvritti from 8.2.82, to indicate that ओम् replaces the whole last syllable, with its vowel and consonant. Had टि not been repeated, then by the rule of अलोऽन्त्यस्य the final letter only of the टि portion of a sentence would have been replaced. Thus in सुम्नयुः, the visarjaniya alone would have been replaced.
When not employed in connection with sacrificial works, there is no such substitution. Thus in simple reading of the Vedas, we should always recite अपां रेतांसि जिन्वति ॥,