PROS:
Moderate damage at ridiculously long range (late game)
Insane damage over time at decent range
AoE spell with great slow %
Both effective against sqiushies and tanks

CONS:
No CC or mobility spell to get you out of trouble (your E is almost useless at melee range)
Little burst damage: You have to stick around to lay out some DPS.
Slow and squishy
Team dependant (due to all of the above)

Attack Speed marks and quintessences are kinda self explanatory:
The faster you attack the faster you can mess people up with you bio-arcane and the most advantage you take from its rather short duration.

Mana regen per lvl seals:
Although not a caster, this build still has enough AP to be able to harass squishies, and sometimes even kill champions with no defensive items. For that purpose these seals will really help keeping your mana up.
If you don't plan on spamming your ult that much or (god forbid) you should choose clarity, you can take some defensive seals like HP per lvl or armor.

Magic resist glyphs:
Staying away from the enemy, you will most likely receive most of the damage from spells so a little magic resist always helps. Magic resist per level is also acceptable.
While ability ability power is not worth it, cooldown reduction and CD reduction per level might just work, but ult spamming will still be limited by mana consumption.

You may also get attack speed glyphs and seals, but I think it's better to stick with primary runes.

Being a hybrid you take advantage of pretty much all the offensive masteries, so I only leave 7 points for minor mana boost and again more magic resist. You can also take the offensive mastery (+ dmg to minons) since last hitting is rather tricky with your early game low AD.
Getting 21 on utility is not recommended for a build that doesn't need CD reduction or mana that much, although lower CD on summoner spells is always welcomed.

MY CHOICE

Imo the best spell for range squishy champions. While you may still be chased, this spell allows you to get out of immediate harm. Works great with your E if you want to get the hell out of there or if you are being focused in a team fight and want to get some distance. Flashing over a wall will almost always secure your escape.

Exhaust works on EVERY champion, and it's just too useful to let it pass. As a mid lane champion it will help you a lot in soloing your enemies, as well as securing the gank if and ally drops by. Since, as said before, your E is useless at melee range, it's also a good defensive spell and will save you from any attempt of tower dive. On team fights, exhaust is best used to shut down their carry (OP undying rage) or anyone who gets to you.

You can also take:

Good chase/escape spell, but flash/E will work as well while being better for positioning on teamfights.

Great spell if being focused, and works good with ghost, but still I prefer flash/exhaust. You should get it only when playing against ashe.

More user dependant than anything, but if you start with lifesteal and have mana regen seals you shouldn't have to back often.

DON'T TAKE:

This is not a spam build, your mana regens seals should do it.

You don't really need any more dps, try to take summoner spells that help you with your ****py survivability. If playing against mundo, however, at least someone in your team should get it.
Although useful early game, you'll then have an insane range ult to help you scout from safety. Let a support champion with utility masteries get it.

Jungle kog'maw doesn't exist......... For now.....

Again, support spell. Plus you don't need more dps and the heal won't save your squishy *** anyways.

Let someone with defense masteries get it.

It's usually best to try not dying, rather than feeding and reviving.

A rather simple sequence, just prioritize W and R, taking void oozle as your second spell.
The only exception is lvl 8 where I get the spittle. Before the exhaust was nerfed it used to be a good close range dmg spell for when you were blinded and couldn't land your W, but now I almost just get it for the bonus attack speed and to throw it on a high armor tank once in a while.

Showed above is the core build leaving 2 spaces (if you sell the vampiric sceptre) for more situational items. The first items are also user dependant.

Early game:

You should always try to get mid lane since you can use the extra exp and money to pwn mid game on team fights. The early lifesteal gives you a great boost in surviving harass, but won't really help you in farming or in sudden fights. Other options are dagger (especially recommended if you don't have the attack speed runes) or doran's blade, which is almost too good early on but you will eventually have to sell it.

Early game farming is not easy with Kog'Maw due to low AD, like all ranged champs, and low dmg abilities. It's still always possible if you focus (and don't get laggy), although sometimes it's better to just auto attack the minions to regenerate HP and get some kills.

Early harassing isn't easy either compared to champions like vladimir or mordekaiser, but if you do it right you can get some easy kills or at least force them to go back and loose exp/gold.
Some Kog'Maws just spam the void oozle for this purpose but that's not effective for a couple of reasons:
-High mana cost
-Rather low base dmg compared to targeted spells
-The trail is to thin and therefore easy to dodge, not to mention it moves really slow
-The slow percentage is wasted

That's why I use the W as a main source of damage while I just throw the oozle to immobilize the enemy. If the enemy still has some HP it's best to throw the oozle directly at him so he can't come closer while you take advantage of your range, however, if you are positive he can't kill you, you can move diagonally and throw it sideways behind him as in to block his only way out. Flash-exhaust is a great combo to finish fleeing enemies too.

Champs like kat, kennen or trynda can be tough since they can easily trample you, in which case your oozle is won't save you. In this case you should try to play a little more defensively and tower hug while getting last hits with your bio-arcane, void oozle or artillery if necessary.
You should also be very aware of champions with blind, like te'emo or corki, since missing your attacks won't allow you to deal your HP%. This is one of the few scenarios where your Q may be remotely useful.

Be sure not to back before you can get at least boots, but you should try to get a recurved bow and build berserker's on your first trip back.

Mid game:

At this point you should be thinking about destroying your enemy and pushing the tower before ganking other lanes. Madred's will help you a lot for that purpose, but even if you haven't yet built then the recurved bow and berserker graves plus your W will do the job. After you get guinsoo's you ability to farm and push will be really maximised and it will only take a minute alone for you to take down the first tower. If the enemy is low but refuses to back, you can always harass/finish him with your ult even as he tower hugs. This is also a great opportunity to take advantage of your W extra range to land some shots on that tower, even if the HP % doesn't apply on structures.
After guinsoo's I usually get trinity force starting with zeal and then phage since I find all of it's bonuses are particularly effective on kog'maw, the least useful being sheen's passive which, even if you have low AD, will help as you spam your ult.
Kog'maw is not a great ganker but if your allies can effectively snare or stun the enemy you will be able to get easy kills. If you approach from the bushes remember to throw your oozle horizontally to block the enemies' way out rather than at the enemy so your slow time will be maximized.
Chances are you won't be able to finish your build since madred's and TE are among the most expensive items in game but once in a while the match will be so long and you will get so fed you'll still earn more gold.

So here are some options for finishing the build:

-Infinity edge: Will give you a great DPS boost along with your already awesome attack speed and crit chance, but again, talk about expensive items.

-Banshees veil: Standard defense item for squishy champs, buy only if you are really getting owned.

-Stinger: Again more attack speed and AP, plus some CD reduction to help you use W and E more often, but your ult is still limited by mana.

-Frozen mallet: Both HP and AD are welcomed, but a little bit pricey and you attack so fast already that you can rely on trinity force's slow

-Malady: Great cheap item for kog'maw. However, since it's a hybrid build, not that useful. Get it only if facing high MR tanks.

-Black cleaver: Same as malady, just against high armor tanks.

-Guardian Angel: Only get if you are getting shut down, but still I prefer banshee's for that purpose

-Phantom dancer: Kinda pricey but still great for obvious reasons. Movement speed will be great to escape/chase.

Attention Do not build any lifesteal items from that vampiric sceptre (bloodthirster, gunblade, stark's). That lifesteal is only for healing overtime when out of combat and will work as such throughout the game just fine. However, no matter which item you build, you'll still be too squishy to be saved by lifesteal/spellvamp if focused, so instead try to spend that money on melting your foes faster or items to avoid CCs (banshee's, quicksilver sash)

Late game/Team fights

With kog'maw it's all about positioning. You have strong dps with excellent range, but no stun if someone focuses you. If used correctly your oozle may prevent enemies from walking towards you, but spells like shumpo or leap strike will get you in trouble. That's why you should always rely on teammates with CC, and even then be cautious on how you approach the battle. Any champ that can snare/stun them or somehow draw their attention from you is all you need. Champs like shen, rammus, amumu, taric or singed are a great help. With a little team-play, you should be able to easily rain acid on your foes and get fed quickly. If an enemy still manages to approach you you'll have to rely on your summoner spells: You can flash and quickly throw an oozle and tear him apart while snared, or simply exhaust him and focus someone else.
Just remember to always leave mana for your R and W, so try not to spam more than 3 ults or so in a row.

That about wraps it up, just remember that there will be some champs you won't be able to easily defeat early game, and I'm not just talking about OPkaiser or vlad: Champions with CCs and nukes or blinding spells will also be a pain in the ***, so if you can't kill them, you can always farm from afar. Better being outfarmed that feeding. If you are new to this champ, you may wanna avoid laneing mid altogether.