Not Updated For Current Season
This build is played very much like a hybrid relying on how hard your Void Ooze and Living artilary hit for the begining of the game, and mowing down enemys mid/late game with Bio-Arcane Barrage.
9 Greater Marks of Insight (Spell Penetration reds)
9 Greater Seals of Clarity (Mana Regen per lvl. yellows)
9 Greater Gylphs of Focus (Cooldown blues)
3 Greater Quintessences of Fortitude (Flat Health Quints)
I go with a 9/0/21 build
Offense: Archmage's Savvy 3/3, Cripple 1/1, Sorcery 4/4, Archaic Knowledge 1/1
Utility: Good Hands 1/3, Perseverence 3/3, Awareness 4/4, Meditation 3/3 Utility Mastery 2/2, Quickness 3/3, Blink of an Eye 1/1, Intelligence 3/3, Presence of the Master 1/1
This is a pretty common Masteries build... Enough points in offense to ger Archaic Knowledge (15% Spell Penetration) Is all you need. Plus the 3% cooldown is a must, the rest of the tree is not going to help much with this build. And 21 in the Utility tree is nice because of the mana regen you pick up, Longer buffs (which you will be getting as often as you can), movement speed, and reduced cooldown of summoner abilities which will make it easier to surive.
I pick Flash and Exhaust flash for escapes and/or catching up to an enemy and exhaust for escapes but mostly to neutralize a melee carry long enough to kill them(Which does not need to be very long).
Red and blue buff are very useful on Kog'Maw and if you are able to be greedy and always have them then do it.
Itemization and Playstyle
Kog'Maw is a popular mid and about 75% of the games I play as Kog'Mog I play mid. That being said I start with regular Tier 1 boots and 3 health pots and put my first point into either Q or E. E if I am ganking and Q if I am not (QEE or EQE are how I spend my points for the first 3 lvls). I go Q because the 10% attack speed right off the bat is very nice and since I usually play conserative until level 5-6 I focus on getting last hits and harrasing when I can. Once I egt my Sorc shoes my playstyle becomes much more aggressive using Void Ooze and Bio-Arcane Barrage in tandum to do as much damage as I can and sniping for the kill with Living Artillery.
Once I have 1800 gold I go back to buy Sorc shoes(750g) and a Madred's Razors (1000g) then head back to laning and ganking. If I can not afford both I will buy Sorc shoes first considering most of my points are going into Viod Ooze. I will usually get a couple kills mid or some ganks during this time along with a ton of monster kills and go get my buffs since Madred's Razor makes it easy mode. Save up 2800g and go finish off your Madred's Bloodrazor. By this point you should have atleast 2 to 3 points in Bio-Arcane Barrage + Bloodrazor = insane damage output. Next I build my Sword of the Divine(2170g) for attack speed plus additional damage, those two items have great synergy together. Here you shouldn't have a problem with any monster on the map and should be able to baron with your team.
Next I build a Stinger(1120g) for more attack speed and to start stacking cooldown reduction, from there a Phage(1315). Next I build a Frozen Heart or Banshee's Vail depending on if Physical or Magic damage is hurting more (I Usually build the Frozen Heart since it gives more cooldown reduction). From here I finish Frozen Mallet and last but not least Nashor's tooth.
-Sword of the Divine or Void Staff (If you want harder hitting spells.)
-Frozen heart/Banshee's Vail or Ryai's Crystal Septer (To give you yet even more slow.)
-If your gamne lasts even longer and everything on the build is complete try turning Phage into a Trinity Force instead of a Frozen Mallet.
Other Possible Items
Archangel's Staff (If frozen Heart/Banshee's Vail/and Ryai's are not ideal)
Atma's Impaler (If you have been stacking health items this might hit hard)
Executioner's Calling (If all the champs on the other team are healing)