So I just defeated the dragon in the windspear hills, and the man told me to go to the Order. I know I'm supposed to meet somebody but I can't find him. Where is he?

Can't recall for certain, but you'll want either Prelate Wesselan at either the top room or the left side room in the radiant Heart, or William Reirrac who is in the large main room towards the right hand side.

Out of curiosity, do you have a save from before you beat the dragon? There's a Paladin quest that you can only get and complete if you didn't kill him. (The quest is to do just that, ironically enough.)

Yes I do. I made a save before him and the shadow dragon in case I ever want to face them again. I would not do the quest, Casyomer(Pardon the spelling) is too awesome.
Thank you Coelocanth, he was in a really obvious place. One question though, I did all the stronghold quests (3), so what purpose does it do now?

Well, each class has their own stronghold and you can only own one even if you are multiclassed. The main purpose of it is to give you a minor source of income and a few sidequests.