Orianna's passive seems to ignore the 30% bonus ap that Rabadons Deathcap grants.
I tried to get 100 bonus dmg with her passive while being lvl 18.
I calculated that I need 350 ap to get to 100 bonus dmg (30 base damage + 0,2*ap).
So I build Archangels Staff and Rabadons Deathcap and I ended up with around 355 ap.
But the tooltip on the passive stated 84,8 bonus dmg.
I attacked a minion to see if the tooltip was just showing a wrong number
but the bonus dmg on the minions was 84 so the tooltip showed the right number.
84-30 = 54, 54/0,02 = 270,
so to get 84 bonus dmg I would need 270 ap which would be my ap
if the passive of rabadons wouldn't count
(155ap rabadons, 45ap archangels, 70ap archangels passive with ~2,5k mana).
I hope this isn't intended to work like this.