Maokai, as a general rule of thumb, is simply your basic tank. This build follows as such:

Meki Pendant + Mana/Health pots
Amplifying Tome
Sinister Codex
Mercury Treads, Unless the other team has no stuns, which is unlikely.
Murillos Evil Tome- Item introduced more or less for Maokai. Gives AP, CDr, and Mana
Regen.
Banshee's Veil- Eliminates any mana issues and gives you that lovely shield.
Sunfire Cape- Fun fun fun in the sun.
About at this point you should be doing okay, so I usually get:
Abbysal Staff- Close range magic penetration is a boon.
Aegis of the Legion- Your Allies will love you for this.

And there you have it. This build should let you cover alot of ground and leaf your enemies in the dust.

Oh, I forgot: Spell selection.
For Summoner's spells, I go for the great all around Ghost and the utilitastic Exhaust. Cast exhaust on any Physical carry as needed.

Usually, for Maokai's abilities, I go with E,W,then Q, followed up with switching between Q and E, getting R and W as needed. Also, you might want to slap down his R before you go in to stun. This lets your teammates know where you are going and sets the battlefield.

He's very hard to build in my opinion. Especially since the first thing you have to do is have access to the shop. Shop OP.

Why would you get the new item over DFG again? If you haven't noticed yet, DFG is basically the same but better for barely anymore cost.

i've found that tear of goddess and FON help a lot. FON + mecury treads = 405 movement speed or else u have the crappy 375 movement speed. and I found that playing tank is more effective than AP cuz ur actually pretty **** squishy

I've been rushing chalice due to being up against so many ap teams. Level up Seedling and the slam, putting one point into root - farming is very important and kindof annoying to do but these two together work very well and i never find myself short on mana.

I then go merc treads and either rod of ages or banshees or omen.

late late game ill sell chalice if need be and replace it with thornmail or warmogs or a zhonya or something.

Lost only 2 games so far out of 7 or so.

AP maokai is so fun. albeit a little suicidal...

I haven't played him =( Gonna drop some $$ to get him though, love the look of his abilities.
Screams to me to be an AP tank.
So I'm thinking,

Chalice
Boots (enemy dependant)
Rod of Ages
Glacial Shroud
FoN
Rylai's

Eh you get the idea, HP, AP, Armor, Magic Resist. Sounds good to me!

If the store ever comes back on, i plan on buying him for sure. I have seen people say he is a tank, an AP tank, and although that is true and how many people will be playing him, I do not see why he couldnt be pure ap. People said renek would be dps tank and for the most part that is how you had to build him till they fixed the health issue. as Pure AP i would imagine you sit back more and throw those sapling things instead of being a jihadist and run in. But who knows i have not actually played him yet stupid store

It helps when you have a team that doesn't overly suck and feed into oblivion. Moakai is very dependent on your teammates taking advantage of his CC. He is super fun when that happens. I tried a couple of different item builds last night on him and they all worked fine but lost some games because of terrible pug teammates. The game I won was against 4 ap champs and 1 EZ so i just stacked MR like whoasnap. Dorans > catalyst > boots > banshees > Merc treads > aegis > FoN > Abyssal scepter > needlessly large rod (almost got a deathcap). Won every teamfight, and moakai can chase down runners very well.

Also found out that the enemy team runs away from your ult immediately, so finding the right time to use it makes it a chore and not fun. Even when the enemy commits to a fight, as soon as you drop your ult, they book it as fast as possible out of it, making it almost worthless.