I have trouble killing people 1v1 with him unless they're stupid (can scare them off easy enough), but that stun makes him a deadly partner for anyone, or if someone else has a controlling effect he can use the ultimate without tagging them first for large damage.
While i am in no way best with Ori. i honestly havent used him in awhile. But for me. its always having your Abilities opens. Thus having the max Cooldown you can have till its maxed out. Then put some ability power/penetrations. and some resistance as well.
Also since your As it is Written refreshes with the other abilities use them in order. i try to use A, X (which ever is the shield), A, B, A, Y, A and if you are lucky your other abilities should be open by then.
I walk alone in the universe as the last of The Time Lords!
the most important thing for ori is knowing when to use which ult. the stun is great for if they're tower diving you or you have some teammates that can dogpile whoever is caught in it, but people seem to only use the stun regardless of the situation from what I've seen which is quite dumb I probably use it about 1/4 of the time the damage the ult can do is massive if someone is caught in it for the whole duration. it's very easy to sneak into the middle of a group of distracted people and just wipe them all out.
also for the gems and relics I like to go full ap it really adds a lot more oomph to him. maybe some cooldown would help but they're not too long as is and you can refresh one of his main damage skills fairly easily.
I really like that one relic sauron can unlock. you don't use an ability for 10 seconds, then the next time you use one you get 120 more ability power on that skill and for 5 seconds afterwards. I use that as my second relic, +70 ap as the first one, and the 14 per sapphire for the last one. for the gems its all +24 ap except the one purple slot and the last one where I just have a 16 cause I am an idiot who likes even numbers and this whole set up gets you at 410 exactly
I max is "X" ability first, Shield second, Ult third and "A" ability fourth.
Sure his DoT gets the highest AP ratio, but its also not on a target longer than .5 seconds before you wipe it off for a more powerful version of any other spell.
I've used relics that simply raise flat AP, or Ability penetration. Reducing cooldowns can help him out as well. The only times I think ive ever done 'bad' with Ori, is when I don't have a lane partner to support my stuns effectively.
I go X first then try to balance out the other two. The A is gets wiped if you use another spell but ideally you hit them with it immediately after anyway, I've gotten plenty of kills after a fight is disengaged because my A keeps ticking on a low hp guy.