The etymology of this word hinges on whether Oscan urust should be accepted as cognate:
* If so, this word is from Proto-Indo-European *h₂er- (“to pronounce a ritual”) (Rix reconstructs Proto-Italic *ōrāō), see also Hittite 𒀀𒊑𒉿𒄑𒍣 (a-ru-wa-ez-zi, “to worship, revere”), 𒀀𒊑𒂊𒄑𒍣 (a-ri-e-ez-zi, “to consult an oracle”), Attic Greek ἀρά (ará, “prayer”), and Sanskrit आर्यन्ति (āryanti, “praise”).
* If not, then a derivation from ōs, ōris (“mouth”) becomes possible; this is still supported by De Vaan.