Kog'maw is a late game champion. This build embraces the idea of being a ***** and farming for gold until the late game. Try to consider your team composition before you choose this champion though, it just might really go wrong. Keep in mind that this build does not work for ADCs.

You'll be surprised to see Talisman of Ascensionas your first item. But this item is actually insane on Kog. It gives him that much needed MS that it craves. Simply turn on its ability and chase or run away. Plus the gold it gives is actually quite nice. Top Kog'maw tends to miss alot of CS during the Early game. Ancient Coin and Nomad's Medallion really helps filling that void of gold that you miss. This is why you cannot go for ADC with this build ADCs are expected to get all the CS with the help of support. Just go getTargon's if you want that late game in bot lane.
Cull is just extra Gold earner.

Your second item will beRunaan's Hurricane.

Then you gothen

Play like a ***** before you get your items. Even if your Jungler ganks, warn your jungler not to go to far for if you get countered, then you're really screwed. Pull lanes. Do not be afraid to throw away turrets in exchange for your life. Your goal is to survive and earn money do not forget that. Try to hold in second turret as much as possible.

If others gank you after they're fed, run if possible. Kite and deal as much damage if not. Just look for opportunities to gain money. The real game starts after you finish your Guinsoo's; poke them till you get your blade then melt them down to oblivion. Depending on situations, this champion can really turn the game around: I had games which we had 8 kills and they had 22. We won out in the end by a lot.