As far as auto attacking goes with any champ in mid, you have to manage the creeps and only try to poke with auto attacks when their creep wave is almost dead. With Ori you can do it quite easily without worrying about the creeps if you take a point in your shield at level 2. Thhe shield will negate most of the creep's damage and then it's up to you to dodge skill shots or trade with your Q accordingly.

As far as champions like Fizz Zed and Nidalee go, Fizz really isn't played AP anymore so you shouldn't really have trouble with him in mid with his changes. Zed is all about your summoners and build priority as Ori. Should pretty much always take either Exhaust or Barrier vs Zed, and prioritize rushing Zhonya's to counter his ult. During lane phase you want to trade with him before 6 and be careful from 6 until you finish Zhonya's unless you get really far ahead. Try to force him to use his Q to farm by punishing him with your Q and W whenever he tries to auto attack to last hit. Ori can clear the waves really fast so if he begins to roam just buy a few wards to help out your other lanes and keep the wave pushed to put pressure on towers.

A matchup vs Nidalee can be both very easy, or very tough. It all depends on your ability to dodge her spears. No player can dodge every spear in lane phase, but again learning to ward properly not only for yourself but also your other lanes will prevent many roams from being successful. Playing vs Nidalee is really about farming until around level 7, when you have some AP and can come close to killing her with a single combo. Ultimately you should try to keep your farm up and not worry about getting kills in lane, Ori brings so much to team fights that the roaming assassins don't that just staying relevant by farming well enough will enable you to change some team fights in the mid/late game.