ओषधेश्च विभक्तावप्रथमायाम्

Operational rule vidhi

In a मन्त्र the final of ओषधि is lengthened before the case-endings but not in the Nominative 1st-Case.

Word by word (padaccheda)

Target ओषधेः ṣaṣṭhī · ekavacanaIndeclinable च avyayaRight context विभक्तौ saptamī · ekavacanaRight context अप्रथमायाम् saptamī · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Indeclinable An avyaya such as च "and", वा "optionally", न "not".
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").

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Full reading

ओषधेः च विभक्तौ अप्रथमायाम्

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

Commentary

In a मन्त्र the final of ओषधि is lengthened before the case-endings but not in the Nominative 1st-Case.
Thus ओषधीभिरपीपतत्, नमः पृथिव्यै नमः ओषधीभ्यः ॥ Why do we say 'before case-endings'? Observe ओषधिपते ॥ Why do we say 'but not in the Nominative'? Observe स्थिरेयमस्त्वौषधिः ॥

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