I enjoy playing Karthus, but for whatever reason i always get matched against an Orianna when i pick him. I tend to do ok early on farming, but as soon as she gets any mana regen she just balls me to death. IT's so hard to avoid, even when i'm under turret. Even if i manage to not die a bunch she gets super farmed and slaughters my team.
Do any Karthus players have advice for playing that matchup?
Not a karthus player, but an Orianna player. I remember one time where I laned against a Karthus and at level 1, he constantly spammed his Q on me without autoattacking to not get minion aggro. Orianna's level 1 Q does very little damage and if she does autoattack karthus, she will get the minion aggro and most likely lose the trade. Then when karthus got level 2, he got E and got all his mana back by autoattacking the minions whenever possible and pressuring me constantly with the Q's.
ok, so level 1 Q spam is a good idea, thanks!
As a 
player, I would like to say that if you are having trouble against Orianna (or any champ in general), ask for a gank from your jungler. If the gank is successful (and you get the kill), you should be able to snowball if you CS well and then you should be able to take her.
Of course, it isn't a good idea to solely rely on ganks. Generally speaking, you should be able to predict when she will harass you with your Q. Try and remember what range the Q has and when she has it off and on cooldown. If she does send it at you, try and harass her with your Q, as she does a quick animation when she activates her Q. Don't forget that your Lay Waste has longer range than her Command: Attack, so try and stay at a distance for when you are trying to farm. If you are having trouble dodging her Q, try building some Magic Resist in the form of 
or 
so that she doesn't damage you as much. If she does get fed at all tell your team to focus her in teamfights or ask for more ganks. Hope this helps ;D
use gp5 seals so you don't need to cs as well.
Use heal early so it comes off cooldown sooner. Spellthiefs edge for first item so you get passive gp10.
Get giants belt for first item.
Survive till level 11 without being too far behind and you generally win.
