kog'maw is a great champion if you know how to build him. Early game you want to keep your mana up and try to conserve it.
Farm, Farm, Farm this is the key to success Early game you want to farm your *** off.
once you get to 6 get your ult and begin to reign hell upon them, WWII artillery style.
at this point your clarity is your best friend use it when you run outa mana when killing someone not when your farming
use a combo of void use then living artillery to slow and kill your enemies.
harass your enemies with living artillery as much as possible

i don't include a rune, but it is fairly simple: mana regen. TO counter your spaming of the ult, get a full page of mana regen

to counter, try to favor ad or ap
if you go for ad get 2 phantom dancers and 2 infinity edges
if you favor ap get magic pent in replace for your infinity edge

the w is your best friend
this will increase your attack speed and rage, allowing you to "secure" the kills ina team fight

kog'maw is a overall op bad *** bug if you build him right
this is kinda squshy but will work