सहिवहोरोदवर्णस्य

Operational rule vidhi

For the verbs सह् and वह्, when a ढकार is deleted due to a ढकार that follows it, the अकार occurring before the deleted ढकार is converted to ओ.

Word by word (padaccheda)

Target सहि- वहोः ṣaṣṭhī · dvivacanaSubstitute / term ओत् prathamā · ekavacanaTarget अवर्णस्य ṣaṣṭhī · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

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Full reading

सहि-वहोः ओत् अवर्णस्य

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

As a rewrite rule (sketch from case endings)

सहि- वहोः अवर्णस्य → ओत्

Replace sahi-vahoḥ avarṇasya with ot. Inherited words (anuvṛtti) may add further conditions.

Commentary

When ढ् or र् are elided, there is the substitution of ओ or आ of the verbs सह् and वह्।
Thus सोढा, सोढुम्, सोढव्यम्, वोढा, वोढुम् and वोढव्यम् ॥ Why do we say of अ or आ? Observe ऊढः, ऊढवान् ॥ Why do we say अवर्णे which includes long आ also ? The rule will apply even when the short अ of सह and वह is changed to आ by Vriddhi: as उदवोढाम्, उदवोढम् ॥ Had merely अ been read into the sutra, instead of अवर्ण then coming after the त् of ओत्, it would have denoted only short अ (तादपि पर = तपरः 1.1.75).

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.