Not Updated For Current Season
When Maokai was first released he didn't seem to get the attention that the majority of new champions get. The reason for this still baffles me. After only a few games with Maokai he not only became one of my favorite champions, but I started winning the majority of my games as well. Up until I started playing him, I primarily played tanky mages in twisted treeline but no other champion does this job better. Although most people play him as a tank, I can't stress enough that Maokai should be considered a mage first and a tank second. However, you can still play the role of primary tank in a 3v3 if necessary.
In solo queue, you should always start with sapling toss. Nothing annoys me more than one or both of my team mates running down to the bush in the bottom lane only to be killed by the opposing team. When the game starts, your goal should be to get a sapling into that bush before any of your team mates get there. If the other team is there, you've done some damage and avoided a gank, if not, then there's a surprise waiting for them when they do show up.
Next you want to put a point into twisted advance. Although the majority of your damage and harassing will be done with your sapling, twisted advance is probably your most useful skill. When you first get it, try to avoid using it unless you need to save a team mate or if the other team gets a little too close. Snaring an enemy right before your team mate comes in to gank can be the difference between getting a kill and the enemy escaping. Trapping an enemy within turret range is always fun too, especially champions like Tryndamere.
By the time you hit level 3 you should know what your next point is going into. The majority of the time I get arcane smash. Using twisted advance, followed by arcane smash can put an enemy in a very vulnerable position. If all goes well, throw a sapling over their head before they can retreat and you've pulled off a nice little combo. If the enemy has been pushing you back to your turret up until this point, don't over extend yourself. If getting close is too dangerous, be conservative and put a second point into your sapling toss and stay out of harms way.
Beyond this, skill sequence is very situational. At level 3 your sapling toss will take out 3 minions at full health, which should be your goal early game. If you're walking all over the other team, leveling up your twisted advance might be a good idea but sapling toss is your primary source of damage and a great harassing tool so in most cases get it to level 5 as soon as possible. After sapling is maxed, level your twisted advance if it isn't already, arcane smash should be last.
How to play Maokai
Although each player will have their own style of play, I prefer to play conservatively on Maokai until level 6. This build is designed for solo queue on twisted treeline, so just assume your team mates are unreliable until they have proven otherwise.
Tip: Saplings can be tossed over several walls, try to harass from the jungle or scout the jungle from the lanes. You can even throw them into the enemy base.
It might be tempting to get twisted advance as your first skill but I would recommend you take the safer route and get your sapling. You know that early game tradition where everyone runs at each other and hopes they get the first kill like a bunch of idiots? Avoid that by throwing a sapling into the bush in the bottom lane before your team mates can commit suicide and most of the time you will avoid a team mate dying 1 minute into the game.
Once minions have spawned, decide who will take the top lane. Maokai can solo if needed but I believe he's much better suited to laning with a partner. Yes he's a tank and that will typically be one of your roles but he's very slow and doesn't have many escape methods early on. An important thing to know about your sapling is that it not only does damage when it explodes but also upon impact. While it may be tempting to harass enemy champions try to stick to farming minions until later. The best way I've found to do this is to wait until the back row of minions gets low on health and then throw a sapling directly on them. If you can time it so a champion is standing in any part of that back row when you throw it, do it. not only will you deal a small amount of damage to the enemy champion but you will either force them to retreat, allowing you and your laning partner to step forward and get some last hits or they will stay and take more damage. Once your sapling reaches level 3, you should be able to clear the back row of minons from full health. Do this as much as possible, without starving your team mate.
If you find that you have enough mana, toss a sapling into a nearby bush whenever an enemy champion is missing. You might prevent a gank, and even if you don't you have a little bit more map control, similar to teemo's mushrooms, but at level 1. Try not to use your twisted advance or arcane smash unless you need to. You only have so much mana and farming is important in the beginning. If you need to step in to save a team mate, do it. If an enemy comes within tower range, use your twisted advance to root them, followed by an arcane smash if you have it. If you think you can kill them with an auto attack or sapling after that go for it, otherwise don't chase, save that for later.
Once you hit level 6 you should start to extend more if you can. You should be able to clear 3 or more minions with a single sapling and you now have all your skills. Continue to harass with your sapling until an enemy has low enough health to finish them. Maokai can deal a significant amount of damage with a single 3 skill combo. Initiate with twisted advance, hit them with an arcane smash to slow them before they can retreat. If they stand and fight, drop a sapling on their head. If they turn and run, throw it over their head to block their escape. If this doesn't finish them off and they're close to death, drop your ultimate on them and deactivate it before they can escape. Many people will save their ultimate for a team fight. Well this is 3v3, if you can kill someone do it. Your ultimate has a relatively short cooldown and is a great way to deliver a last hit. Most of the time one of their team mates will come to their aid by this point anyways.
By this point in the game you should be pretty difficult to kill. No need to be conservative now. Stick with your team mates and if the situation presents itself, initiate a fight with twisted advance. If your team mates have any common sense they'll capitalize on your actions. If you can't get close enough, harass with your sapling and continue to scout bushes with it before your team enters them. If a team fight breaks out, wait until everyone is committed and throw down your ultimate. Do your best to contain the enemy within your ultimate using twisted advance and arcane smash. If the enemy tries to leave, deactivate your ultimate before they get away so they take some damage. If you notice that you can kill their carry by deactivating your ultimate then do it. Don't underestimate the advantage of killing off an enemy champion dealing massive damage and dealing damage to all enemies remaining in the area. As always, try to focus the weak champions, if they try to escape... snare them.. slow them.. kill them!
Many Maokai players will insist on using tank runes, but I've tried this with little success. I switched to my usual magic pen / ability power rune page that I use for Morgana and other mages and it made my life a whole lot easier. Using the runes listed above combined with a Doran's Ring will give you 33 ability power, 8.5 magic pen, and enough mana regen to outlast most champions in your lane. on top of this you'll be getting another 45 ability power by the time you hit level 18. In my opinion, your defenses should be dependent on who you're facing anyays, so leave that to your items.
Just like most people recommended runes that are typically used for tanks most people will suggest that you put 21 points into the defense tree of your masteries. Once again I disagree with this. Using a typical 9/0/21 mastery build is much more beneficial. Something you will notice very quickly is that Maokai is very mana hungry. Using the utility tree combined with a Doran's Ring will allow you to stay in your lane much longer and have shorter cooldown times. While cooldown reduction isn't needed as much in the early stages of the game, later on it can be the difference between landing another twisted advance while chasing an enemy down and letting them get away. The speed your gain from the utility tree is also helpful, considering you are one of the slowest champions in the game.
Ghost and Clarity. These are the only 2 summoner spells that I really recommend. Maokai is slow and mana hungry, and these will remedy that. Some will argue that flash is better than ghost but I find it unreliable. Clarity will not only allow you to stay in your lane longer, but can allow you to let your ultimate run almost twice it's normal duration. You thought your ultimate would turn the tides of a team fight? Why not make it stronger.
Now for the fun part, the items. The item build that I selected above is a general outline of what you should be aiming for. Strictly following that build will make you a formidable opponent. However, every game is different and each item should be chosen for a reason.
I recommend starting with the same 3 items every game. First get a Doran's Ring, then get your boots, then get your Rod of Ages. Your boots will vary depending on who you're facing.
Use these if the enemy is heavily based on ability power or if they have a lot of cc.
Lots of AD enemies? These might be a good choice.
You already have a tanky team and you want more damage? You'll hit that much harder with these.
After you've completed your Rod of Ages you need to make some decisions. Mainly, what defensive items do you need. If you're being targeted or just struggling to survive long enough to get kills you probably need to buy some defensive items. If you're playing more of a support role or doing the majority of the damage, get some more ability power and magic pen items.
Vs. Ad Champions
Frozen Heart is a great item when facing Ad based champions. Actually, it's a great item for Maokai regardless. The extra mana and cooldown reduction are invaluable and if you're facing Ad champions, this item will give some extra armor and slow down those auto attacks.
Sunfire Cape is never a bad choice for a tank. One of Maokai's goals in a team fight is to keep the enemy within his ultimate for as long as he can using his twisted advance and arcane smash. Why not do a little extra damage while you keep them there in addition to making yourself that much harder to kill.
Thornmail can be useful in some situations. If an enemy champion is bursting you down with auto attacks faster than you can kill him, Thornmail will most likely cause them to kill themselves faster than they kill you.
Vs. AP Champions
Abyssal Scepter isn't the first item you would normally think about for defense against ability power champions, but remember, Maokai is not just a tank, he's a powerful mage too. Providing magic resistance, ability power, and magic pen, this item is a must have in almost all situations.
Banshee's Veil is a standby item for magic defense. Consider it the defensive counter part to Rod of Ages, everything it provides is helpful. More mana, more health, magic resistance, and one hell of a passive.
While most players will choose Banshee's Veil as their primary defense against ability power champions, I typically prefer Force of Nature. As I've stated before, Maokai is slow, and the extra move speed can make a huge difference. The health regen is not to overlooked either, combined with Maokai's passive it can allow you to continue pushing after a team fight rather than retreating back to your base to heal.
Ability Power Items
Your build is based around this item. You want to get it as early as possible in order to get the full benefit from it's passive sooner rather than later.
Most defensive items should be considered before this item since you already have a snare and a slow, but it's still a great item. More life, more ability power, and a nice passive. The main benefit from this item is being able to slow your enemy with your ultimate, making it harder for them to run from it, or slowing them with your sapling. It will make it much easier to finish them off after harassing if they're easier to catch.
It's worth mentioning again. Not only is it a great defensive item, but it will provide you with additional ability power and magic pen. Magic penetration should not be overlooked. Combining Sorceress Boots, Abyssal Scepter, and your runes can have a devastating effect.