This is a build I used to great success in a 3v3 match; coming from losing both inhibitors before we pushed a tower to repelling their every invasion of our base, scoring multiple assists before we eventually pushed back and won. I don't know how well (if it will at all) it will translate to 5v5, or how to successfully use Maokai in 5v5, so I'll stick to 3v3.

Update: just finished a 5v5 with Maokai as the tank for our team. Went AP/CDR as usual, but also picked up a Warmog's and FoN for a little more survival against Yi and LeBlanc. Went 7/9/19 and we won.

I'm not going to go into runes, masteries or Summoner Spells in great depth, nor will I bother explaining Mao's spells; they're all pretty self-explanatory. Runes are pretty standard, masteries help you take more damage since you'll be at the center of the fight, and summoner spells are mostly preference - I just prefer Flash and Clairvoyance.

I've seen some builds advise leveling Arcane Smash or Sapling Toss during early levels, but Twisted Advance is where it's at. The increased stun duration will pay huge dividends in fights and let you set up Arcane Smash to knock the enemy champions to your team or into a turret.

The item sequence above is my base build for most fights. While both gp5 items might seem like overkill, both can build into good defensive items later. The second tier of boots is left up to your discretion; obviously, Mercury Treads are a safe bet against CC, but Ninja Tabi might be worth it against AD-heavy teams.

I don't buy more AP items beyond the Tome because Maokai's scaling is pretty bad, but his base damage is amazing, so getting more CDR will let you dish out the damage instead of building AP. The defensive items you get instead will let you survive longer, building a better Vengeful Maelstrom while giving you more time to punish your enemies.

The sixth slot is left open as needed, whether it's more armor in the form of Sunfire Aegis or similar (the DoT is also good, given the amount of time you can W). Frozen Heart will give you 40% CDR (and a 7.8sec CD Twisted Advance) and a good chunk of armor and mana, as well as a good aura.

Abyssal Mask is also a good 6th slot item to simultaneously boost your damage and lower the team's MR.

I know this is brief and not too assertive, but this mostly serves as a self-reference, as well as maybe some help to others looking to put the tree mage to good use. If you have any comments or criticism, please do not hesitate to tear this apart.http://www.mobafire.com/league-of-legends/edit-build?edit=91703#