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o- (2), ob- Prefix "toward"
From Latin ob-, a grammaticalization of the preposition ob 'id.' From Proto-Italic *op 'id.' From Proto-Indo-European *h1opi̯ "at." Perhaps an ablaut of *h1epi̯ meant "on."
Indo-European
Italic
Oscan úp "at," Venetic op "from"
Balto-Slavic
Lithuanian api-
Hellenic
Ancient Greek ἔπι (épi) "on," Mycenaean e-pi- 'id.'
Armenian
Armenian ew "also"
Indo-Iranian
Sanskrit ápi "by," Avestan aipi 'id.'

On the word *h1opi̯, Sihler (1995) writes, "Some see in these forms various cases of an otherwise lost root-noun meaning literally 'back.' The 'on' sense may have something to do with pack animals or riding."