Remember not to run away from your enemies when you die, as Icathian Surprise will allow you to deal extra damage.
Think before Flashing away from a laning phase fight; if you notice that the enemy marksman is tower-diving you and you're about to die, keep striking him while getting as close as possible. When you die, follow him with your Icathian Surprise, possibly getting a return kill or burning an enemy Flash.
Use Void Ooze's slow before a Bio-Arcane Barrage charge. Maxing your Bio-Arcane Barrage can be a good way to get kills early game, or simply bully your lane opponent. The percent health damage on it is one of your greatest advantages, allowing you to take down even tanks.
Void Ooze is Champion only disengage ability. In order to succeed in running away, point Void Ooze away from your enemy towards where you are running. You will be able to run over it without a problem, but they will be forced to run on top of it the whole time resulting a drastic speed reduction. Ghost or Flash can give you the "lead" needed to compensate for the short casting time.
You can however, click on the map immediately after using Void Ooze to stop the animation, making the escape easier, though it can still be dangerous and Ghost or Flash can compensate where necessary.
Ability Power, combined with Living Artillery, can lead to devastating long range damage. This is a great way to prevent your enemies from camping by a turret.
If the Living Artillery hits a champion, your team will be able to see that champion for 4 seconds. This can be useful to continue auto attacking or using other targeted abilities after an enemy has fled into the bush.
You can use Living Artillery behind the enemy to scare them so that they run towards you, which can then let you land other shots.
On Howling Abyss, Kog'Maw can deal massive amounts of damage and harass using Living Artillery and Void Ooze provided he has built AP. Void Ooze is particularly effective there, as it has both long range, AoE, a very good ratio, and a slow which will help you land subsequent Living Artillery shots. The only downside to Void Ooze is its relatively long cooldown and slow projectile speed, but this drawback can be mitigated by the fact that most players do not see it as a real threat when it's not used to block off their escape, allowing Kog'Maw to unexpectedly deal high amounts of damage with it. As Void Ooze hits all targets in a straight line, it can also be a good tool to punish the enemy team for camping in bushes.
Kog'Maw is a flexible champion with a variety of build options due to his mixed damage and scaling, similar to Ezreal. As an ADC, his kit is more auto-attack oriented due to his Bio-Arcane Barrage and the built in attack speed passive on Caustic Spittle, thus he scales better with attack speed, critical chance, life steal and on-hit items rather than flat attack damage and armor penetration items. As an AP mage, he makes use of all of his abilities (particularly his Living Artillery ability) to deal heavy sustained damage from afar and significant instant burst in moderate range emergencies.
A good default Kog'Maw build has several AD items, one serious mana regeneration item, and assorted hybrid helper items according to taste that give AP, attack speed, life steal, cooldown reduction, or other stats appropriate to the situation. However, he can also be built pure AP, which reduces his CS ability and single target damage but strongly improves his burst and the damage of Void Ooze.
When building a hybrid Kog'Maw, be sure to get a Guinsoo's Rageblade. It is one of the most cost efficient items on any hybrid build.
Getting some life steal at the beginning of the game is a good idea to help you survive in your lane longer.
Starting with a Vampiric Scepter and building into a quick Bloodthirster can lead to some great damage output for a DPS build. The shield is also very useful as a defensive component as Kog'Maw is naturally very vulnerable and oftenly a top priority for the enemy team.
Rushing Trinity Force is relatively useful for Kog'Maw since the Phage's Rage passive adds additional mobility for kiting and chasing potential, something that Kog'Maw lacks. Its mixed stats will also make Kog'Maw's abilities slightly more potent.
Building AP or DPS depends on what the enemy team consists of. Tanky DPS and tank enemies will call for DPS, while a balanced or assassin team will need AP. This is because DPS will kill faster but expose yourself to danger, while AP lets you take off health from a reasonably safe distance.
However he is very vulnerable to all-ins. Supports who can perform such early on like Thresh, Leona or Braum are very effective against him
Properly building defensive items againstKog'Maw is incredibly difficult. His damage is very high on both sides while also having % max health damage and a armor & magic resist shred on top of that. Almost no champion -tank or not- is able to take his focus for too long, so locking him down and killing him as fast as possible in any fight is a top priority. Some champions who can force are Vi, Zed, Fizz
Kog'Maw is fairly squishy at early game. Putting pressure on him can make him under-leveled.
Kog'Maw is a champion who requires a partner with strong CC to keep him protected.
He is one of the few ADC with no reliable escape ability, so he is usually paired with highly defensive supports. He is still very vulnerable to ganks compared to most of other marks(wo)men
As with any marksmen any bursty mage or assassin can quickly kill him past the early game. This is very important due to the sheer damage menace he represents
Never feel safe when Kog'Maw is somewhat near, Living Artillery has a ridiculously long range and can harass and even kill you while on a turret. This is particuly important against AP Kog'Maw, as -unlike marksmen Kog'Maw- he can sustain the usage of the ability for a very long time thanks to more wide mana & mana regeneration items available to mages, and the ability power ratio of the spell is significant considering it's cooldown and cost.
AP Kog'Maw is one of the best turret siegers in game. Assasins and/or champions who can rival his range (Xerath, Ziggs) minimize such threat. Sivir also well known for being one of the best (if not the) siege breakers in game thanks to her powerful ultimate that wides enormous team-wide speed up.
Kog'Maw is extremely vulnerable on his own and if left alone he is a very easy gank.
Be very careful with early battles, bringing down Kog'Maw first will often result in you getting killed by Icathian Surprise, try to kill his support first to scare him away, and proceed to overwhelm him, otherwise the support will take the chance of killing you with his/her own damage along with the potent blow of Icathian Surprise.
Kog'Maw can win all AA trades due to his massive damage boost, only a few marksmen like Draven or Varus can match him, always avoid direct confrontation and step in for the kill after you have poked him under half his health. You can also overwhelm him with burst supports like Blitzcrank and Morgana before he has a chance to properly damage you.
Dashing or blinking from his ooze will make his slow useless, but avoid using speed boosts while in the slow as they will not give you the speed needed to escape.