ओसि च

Operational rule vidhi

An अदन्त अङ्ग gets a एकारादेश in presence of the ओस्-प्रत्यय.

Word by word (padaccheda)

Right context ओसि saptamī · ekavacanaIndeclinable च avyaya

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").
Indeclinable An avyaya such as च "and", वा "optionally", न "not".

Dotted words are technical terms, dashed ones pratyāhāras (names for classes of sounds or affixes). Hover or tap one to see its card.

Full reading

ओसि च

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

Commentary

Before the case-ending ओस् , ए is substituted for the final अ of Nominal-stem.
As वृक्षयोः (Genitive dual) स्व, प्लक्षयोः स्वम्, वृक्षयोः (Locative dual) as वृक्षयोर्निधेहि, प्लक्षयोर्निधेहि ॥

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