Hey guys, so where should I begin... Well I've been playing Kog'Maw for about a year now, so I decided to make a guide to show the others how I think Kog is supposed to be played. There are three basic ways of building Kog - the one from the EU(Infinity Edge), the one from the USA(Madred's Bloodrazor) and ap Kog. I am sorry, but I'm not gonna show you the madred build, because right now ( since his ultimate got ad scaling) the Infinity build is so much better.
If there is going to be an update I'll let you know ;) So this guide is going to be focused mainly on the Infinity Kog and a bit on AP Kog. Have fun reading and owning rankeds!

The Path of the Infinity... hmmm... sounds noble huh doesn't it? I think your opponents won't find it anyway noble of you to slice them in 3 strikes from the 25th minute...
The runes I use when I'm playing the Infinity style are:
9× Greater Mark of Desolation
9× Greater Seal of Vitality/ 9×
9×
3× Greater Quintessence of Desolation
This gives me about 25+6 armor pen which is pretty cool and will allow you to slice your enemies from the beginning. Also it gives you some Magic resist and Health/Armor so that you when you go bot with some healing support there is almost no chance of you dying early.

I take pretty simple ad carry masteries - Remembering to take Improved Exhaust, Increased Exp Gain, Armor pen, Crit chance, Crit damage, Cooldown Reduction, and one point in Mana regen.
All of these masteries are highly recommended.

The Items I take.. well there are times when I experiment, but most of the time i go:
-> -> B.F. Sword -> -> -> and from them I usually go
and then I take another or (optionaly both if the game is getting long enough). You should be able to take your I.E. in about 20 mins (15 minutes are the best you should try to achieve.) After the that it gets quite easy.

So my Skill Sequence... You are probably asking why the hell I am not taking a single point in Void Ooze until mid game. Well this was a tough time deciding wether to take at least one point in it or not. I must say that personally I'm taking one point sometimes (usually when I'm dying early - Yep that happens) but otherwise I do not since Caustic Spittle is for AD Kog'Maw much better choice. I max Bio-Arcane Barrage first, maxing Caustic Spittle second but taking one point in Void Ooze at lvl 8. And as always taking my Ultimate whenever possible.

For summoner spells, I always take and . It is so good as Kog you can't even imagine... Oh well... You will be once you play Kog few times... :D
Reasons:

This is the best spell for Kog'Maw - First of all it is extremely well combined with his ultimate, second it's the best of the summoner spells for escaping and third you can use it to jump behind the wall, using R (so that you see your enemies) and killing them with Bio-Arcane Barrage.

I've been taking exhaust every game since it got buffed a few months ago. It prevents you from dying against most of the champions and it even buffs your damage while improved (Which combined with Caustic Spittle and runes reduces the Magic resist and Armor of your enemies down to zero most of the times).


Other used summoner spells

I've seen a lot of Kog players taking Teleport, but I don't really find that needed since Phantom dancers give you enough speed to travel through the map fast enough.

Traditional Summoner Spell used often with Flash to reach the most effective technique of escaping.

I find this spell used only by noobs that can't hit anything with their ultimate (using this only to kill their enemies once they are about to escape).

Void Ooze is one of the most effective slows in the game, so it has many ways of casting it.

1. You are in a team fight - cast it right into your opponents so that they have a tough time getting nearby.
2. Lane usage - cast it in front of your target covering most of the lane if you are as far from the turret as he is - if not cast it right at him.
3. If an enemy is chasing you and he is right behind you, cast Void Ooze right in front of you - just the way you are going to run.
4. If an enemy is chasing and he is at least half the distance of your maximum range of your Void Ooze - cast it right at him.
5. If you are playing ap - spam it as hell and you will see how they die.

Living Artillery is one of the hardest spells to cast in League of Legends - Kog'Maw may be found quite easy to play - but using this ability well might become learning him kinda challenging. You can use it easily to last hit minions or scouting the bush - but hitting a running enemy right in his face while he is skilled enough to dodge or spamming the ultimate just so that you can attack few times before using it again takes hours and games to learn and a bit of gameplay skill too.
If you wanna hit your opponents properly, while playing Kog the first 20 times you are highly recommended to turn off the music and take at least 2 minutes spamming the ultimate and listening to the sound. Quite a weird tactic but it helps really much. Can you hear that "Blaaaaah boom" sound? Remember it well because if you do you can easily know when your ulti is going to land. Therefore you can hit your enemies more often or always and you are going to be a real pain in the ***.

You start of with Doran's Blade and you should head bot having one of these champs by your side:

I usually try having Taric on my team - since he is the Number one with you. Tell him to pick some wards and heal and stun and you are just ok. You should try farm minions and harras opponents with brutal dmg from Bio-Arcane Barrage. Once you hit 6 take your ultimate and harass like a boss doing auto-attacks and spamming your ultimate but keeping some mana so that you can kill them if they get greedy/stupid/garen complexy. If you are fed take fast B.F. sword and boots of speed. If not... well... farm and take it too ;) After grabbing B.F. and boots you should be just owning. Farm few more creeps, few more kills and afterwards take your Infinity Edge. You really need to practise using your ultimate while auto-attacking (that will allow you to pick easy kills and farming your Dancer.) Once you get your Phantom - you are now having Awesome attack speed, Awesome Damage, Awesome Crit. chance, Awesome everything. Try pushing your towers with your long range attacks but don't do that if you feel it can turn out bad. Kog'Maw is the best for his long range - he can kill 3 guys under turret with almost full hp if fed. Try doing that and a good score is guaranteed.

As I said in the upper part of the guide - I will be discussing AP Kog a little bit too. The reason I love Kog'Maw as AP as well is his ultimate. With it dealing 150% dmg against champions and having about 1 second long cooldown, and it having such an OP range... just... AWESOME!
AP Kog'Maw isn't a really good burster. He excels in long range fights and while maxing Voide Ooze and taking Rylai's Crystal Scepter in keeping his distance too. So as AP Kog your main purpose is to get fed. Once you do that (and trust me you will) you can slice your enemies with your lethal magic damage. I don't recommend you taking AP Kog'Maw till you get to know your ultimate and you hit more often. This build is optional so that's the reason I don't really discuss it - and it doesn't really have a tough gameplay so you only gotta read this and the stuff on the top of the guide.

Summary:
Max Void Ooze, Take AP stuff like Rabadon, Rylai and Archangel and harass your opponents with your ultimate. Keep the blue buff on since your ultimate costs a lot of mana to cast more often. Remember your "q" also reduces the target's Magic Resist therefore it is well combined with your spells. Use your ignite to finish ppl off and if you die finish them with your Passive too. Your last pick should be Madreds Bloodrazor since you are then supposed to deal lots of dmg with your autoattacks too (Yes, It has an AP scaling).

Thank you all for turning into my guide for Kog'Maw the Mouth of Abyss. Please tell me anything you find bad as far as this guide is concerned. This is my first guide but I don't find it that bad myself.. Again, Thank you all!