they are equals. think about it.
base - kusanagi, clones, snake summons vs frog summons, toad oil, barriers, rasengan, hair needle senbon.
boss summons - manda and rashumon gate defence vs gamabunta, gamakichi, toad gourd prison
high level - true snake form, hydra mode vs sage mode and genjutsu frog song.
imo oro in true snake form poisons jiraiya with his body, then spits kusanagi out of his mouth and stabs him in the face.
but if jiraiya starts in SM then oro will use hydra mode which shrugged off having its heads cut off by totsuka, and use rashumon gates to block jiraiyas hair needle senbon
if worse comes to worse, orochimaru can destroy jiraiya by using first, twin snake mutual assasination then use substition or a clone so he wont die too.