Obviously outscale , but is 
rush a good idea ? New to top , just playing around other winning lanes and trying to stop the bleeding and not feed in these hard matchup's
What should you rush against an ap counter as well ?
You, my friend, need to learn wave management/manipulation. The sad truth is that, without Q scaling, you will likely never out-duel a Renekton or an Olaf at early-mid game.
With that said, against Olaf, armor/attack speed reduction work pretty well. Building into Thornmail first is not a bad idea. I’d recommend rushing it along with Frozen Heart right after and Ninja Tabi, since most of his damage is physical and auto-attack based. The cold steel of Thornmail stacks with the aura of Frozen Heart for a 30% attack speed reduction. That, combined with your Wither (so long as he’s not using his ultimate, which renders him immune to disables) should allow you to compete.
Renekton’s a bit of a different story. You’re gonna have to play safe against him due to the dashes and free armor penetration he gets. Build more health against him, and save your wither after he blows his dashes. He deals ability based damage, so Thornmail is not as effective on him in my experience. Freeze your wave in front of your turret, help your jungler with ganks, scale up, and beat him down.
The answer is you shouldn't.
That's like asking how you beat a pistol with a longsword. The answer is to call your buddy who has a pistol. (ask for jungler help)
I read "how do I beat off renekton as Nasus" and got extremely confused
Stay under your turret and play like and ultra pussy for the first 25 minute of the game. You will be rewarded fir it by becoming an unkillable monster.
You hope they get a massive seizure.
Play safe. Let them push the wave and freeze ot by your tower so you can be safe.
