Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
as an ap class i would go the 9 points into offensive for the extra spell pen i get ghost because when i play i feel kog'maw is slow and the increased move speed helps more then if i had got blink. i dont get any mana regen talents because when i play i would normally get blue rune when i can and it is nearlly impossible to run out of mana with it.
i get void ooze first because it has a longer range then caustic spittle which i dont get untill later because i try avoid being into close combat. i get bio arcane barage as my other main skill because the increase range and it still helps deal alot of damage.
i get magic pen red runes because they are the primary for red and they will increase the damage of all the spells which will increase your outcoming damage. i get health and health per level seal's and quintesses because kog'maw is very squishy and the extra health helps alot early game (if you want you can get health instead of health per level seals but i prefer the extra late game health). and i get cooldown glyphs because it increases the amount i can spam my ultimate.
i get a doran's ring first because the mana regen helps me spam my skills faster and a health potion incase i need to heal myself after taking some damage. i normally like to get a mejai's asap because i want to start stacking it when i can but if u cant afford it after your first base i would get the amplifying tome and boots of speed for the extra mobility. i get the sorc boots for the extra spell penetration which helps and the movement speed so i can escape easier. i get archangel's staff next because i spam my skills alot and it has alot of mana regeneration incase the golem is dead also gives lots of mana which is needed if you dont have the blue rune. next i get zhonya's ring becuse it gives a massive damage bonus. i get the rylai's scepter because it gives lots of health which increases survivability (also you can kill people running using your ultimate only :P) last i get lichbane because it gives 7% movespeed and even though your ap you have the bio arcane barage which will be used giving you a massive damage bonus on your hits (you can sell your dorans ring to get a sheen if you want to get it i personally do because it gives more ap every little helps). also i get all the elixirs because you have nothing left to spend on and thanks to a patch they automatically activate if your bags are full
hope you like my ap kog'maw build enjoy :) ( dont say its bad if you havnt tried it without a bad people ofcourse or thats unfair judgement)