When versing Orianna main thing you need to focus early is pushing the wave swiftly while avoiding to take poke and optimally dealing some star damage as you're hitting the creeps. As long as you dont take extended trades, with the corrupting potion, you should be able to out sustain Orianna regardless of what she has bought and took as rune. Unless you have electrocute and ignite, don't look to kill the Orianna but rather deny/provide vision, be first to rotations and make roams as well as drags happen with the priority you hopefully secured. It is not an easy matchup by any means if the game is slow paced and is scaling centered, however, as long as you manage to create a CS diff by push&poke swiftly and ideally secure kills&assiss with the roaming power at hand, the MID DIFF shall be insurmountable.