Such poor logic its cringe worthy.
We havent seen any jutsu from hiruzen which suggest he was op?, how do you even tackle such a phrase?
. First of all op jutsu's take a huge portion of your chakra, old hiruzen doesnt have that much chakra, regardless if summoning enma (one of the best if not the best summoning in the series), but also using the reaper technique isnt impressive i dont know what is.
The highlighted post of yours is rigged with mistakes. You say 'But op? i doubt'. Thats an your assumption and nothing more. You say onoki is much older than hiruzen when he fought oro, where did you get this from. Hiruzen fought oro at 69, how old do you think onoki is?. So just because onoki has shown more potential in his old age than hiruzen, he mustve been op in his prime?, such nonsense. Ninja's mature and age differently. Id go as far as saying prime jiraiya was when he fought pein at 54. Secondly people age differently, just because Onoki seems stronger in his age than sarutobi doesnt mean he was op in his prime, not at all. You seem to forget that Onoki has stayed tsucikage and is still the current tsuchikage. Sarutobi retired and with that, took a break and the pressure of being kage mustve lifted of him and with this he did not need to become stronger.
Prime Onoki stronger than prime hiruzen?, such nonsense.