Here's my build to AP/tank Maokai. You can be really tanky and do lots of dmg at the same time with this. You're also a great support type tank, because of your ultimate and the Aegies of the Legion, if you choose to build it.

Magic Penetration Marks - These help you to kill a lot faster, they also give you some early magic penetration to make up for not getting Sorcerer's Shoes.

Mana Regen Seals - These can be replaced by mana regen/level, depending on your preference. Some extra mana regen really helps Maokai a lot.

Ability Power/Level Glyphs - These help you to hit a lot harder mid-late game.

Armor Quints - You're a bit low in armor with this build, these help make up for it.

I go heavy on the defensive tree. This is good because you're a tank, obviously. If your team is already pretty tanky, you can probably get by with going utility masteries and play more as an AP.

I go with a Doran's Ring. Mana regeneration is a must on Maokai and this does the best job. You could also go with a meki pendant or maybe a mana crystal and possibly build it into a Sheen. I like the ring though for extra health and ability power.

I usually get Boots of Swiftness for the sake of initiating and chasing. These can easily be replaced with something else though and you should get whatever the game calls for. Some good alternatives include-

Ninja Tabi - You have the speed boost per dodge mastery, making this a solid choice. This is good vs teams with strong dps champs.

Merc's Treads - Get it vs teams with high amounts of CC.

Sorcerer's Shoes - If you're building more as an AP or someone else on your team is doing most of the initiating, you should get these.

- Rod of Ages is the perfect starting item for Maokai. The mana, health, and AP is all great on him. The best part of this item is the passive though. The health/mana regeneration, along with your passive, allows you to stick around in your lane pretty much forever with almost-full health.

Aegis of the Legion will give you most of the armor and magic resist you need. It's a great support item for your whole team too. If someone else on your team is getting this, you can go with a Guardian Angel instead. It's a little more expensive, but it has a great passive.

- Abyssal Scepter gives you some extra magic resistance as well as more AP and magic penetration. Overall, it's a very solid item and a great choice.

- Rylai's is an awesome item for Maokai. A slow for your sapling toss, health, AP. It fits with the tanky ap really well. You can get this before or after the Abyssal, depending on whether you need the slow or the MR first.

- Rabadon's gives you lots more AP. All your other items, along with runes and masteries, should get you tanky enough so that you can just go with this for a pure AP boost.

Other Options-
- If you need to be tankier, Warmog's is the item to get. You already have plenty of MR and armor, but you can always benefit from more health.

- CDR is always nice. It allows you to Q,W,E more in team fights and maybe even use your ult twice. The mana regeneration is also good. Get this item if you're really spamming your abilities and for more cc.

- This is similar to Morello's, but it doesn't give you the AP. This is a tankier choice for CDR.

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I have Ghost/Exhaust in this guide. It's great for chasing, fighting, and all that. You're also basically impossible to kill with both an extra slow and a speed buff. These, along with your stun and knock up, make you extremely hard to gank successfully during the laning phase. Other good choices include Ignite and Fortify.

Use W on the carry, hit as many as you can with Q, then use ult and E. Spam your abilities as fast as they recharge and try to keep the team fight in your ultimate as much as possible. Set off your ultimate either when the fight is leaving it or when you can finish off the other team's carries with it. That's the jist of it. You can use ghost to get in range for your W in a timely manner, if need be. Then, use Exhaust on anyone who tries to run away, but remember that it's more important for you to protect your carries than it is for you to chase their team.