Before a pit sārvadhātuka affix beginning with a consonant, the final u of ūrṇu optionally gets vṛddhi ,
As प्रोर्णोति or प्रोर्णौति, प्रोर्णोषि or प्रोर्णौषि, प्रोर्णोमि or प्रोर्णौमि; but प्रोर्णवानि before an affix beginning with a vowel.,
ऊर्णोतेर् विभाषा
Adhyāya 7 · Pāda 3 · Rule 90