I am no professional player and I just bought Cho'gath yesterday. Here the build I was thinking going with, to make an offensive Cho'gath, opposed to the tanking Cho'gath that every build I see are using. Please leave your comment.

I'm using the same rune that my Vladimir is using, because I don't have enought point to buy new one yet.

Magic Penetration: since most of Cho'gath abilities are do a lot of damage, if the opponent got even a little bit of magic resistance, it will cut a good amount of damage.

Ability power: rupture take 100% of the AP, feral scream take 70% of the AP. Feast use 50% of the AP so having a good amount of AP can be nice.

Vitality/level: it become better then flat HP at level 5.

Quintessance Flat AP: to increase the chance of a first blood by doing 15 more damage with Rupture

I took a 9/21/0 build. 9 in offensive to get the 15% magic penetration to lower even more my opponent defenses with Exhaust and reducing the CD of my abilities.

in the Defensive tree: nothing do less damages then a dead Cho'gath, so a minimum of defensive abilities can be nice. and Ardor is alway a nice ability

I start with the mana Gem since it doubles my mana pool, so even if I play agressive I get OOM around level 5 instead of before level 2^^

Then I get the cataclysm gem to heal myself when I level up.

Then the mercury boots to increase my speed, and reduce the effectiveness of spells and CC. If the opponent team have no CC you could swich it for Ionan boot of lucidity (CDR)

my 3rd item I go get my Rod of the ages so it will have a lot of time to charge up.

Guardian angel: it always nice to but fear in your opponent with that aura around you so they do not touch you knowing that you will raise back up (who attack the tank first) letting you do your damage freely.

Rabadon deathcap: 155AP + 30% of your AP... mega huge rupture damage on everyone stupid (or blind) enought to stay in it. and my feast do a little bit less then 1000 damage on heroes.

Banshee Veil: well a nice protection that every character who uses mana should have unless the opponent team got no spells.

Then we'll end with Abyssal scepter for a nice AP boost, magic resistance boost and give all my friends magic penetration. For sure if your team have no spell user you could take Raylai scepter instead, but remember that the slow effect will only slow down the movement speed by 15% since all your ability are AoE abilities (except feast, but the target should be dead after feast)

Rupture first, nice abilities to instill fear in your opponent, and with some practice you should be able to force them to stay out of XP reach.

Second I take feral scream so those brave enought to approach will get silence and should run away fast enought.

3rd level is Vorpal spike, what is more terifying then a big monster that can shoot spike from the ground and silence you? it's a big monster that can shoot spike from the ground, silence AND shoot spike every time he attack a minion.

Then rush to get rupture to max level for maximal damage output (except for level 6 where you'll take feast to become even BIGGER)

I use Ghost to I can run in or out of a fight easily.

Exhaust, if I can reduce the damage the opponent do, reduce it's running speed and lower it's defense (with the talent in offensive tree) why not?