Warmog is a bad item in general and even worse on Cho. You get most of the bonus hp you need from Feast.

what runes/masteries do you bring for this build?

The biggest options are:

9/0/21
0/9/21 (This is the one I use)
0/21/9

My runes are:
Flat Health Quints
Mana Regen Per level Yellows
Magic Pen reds
CDR blues

Other people prefer tankier rune setups though.

Just dont rush warmogs,lol.

Warmoogs is not very good with chogath... even with ZERO HP items he can get close to 3000 HP.

Also there are a LOT of builds for chogath. I'll post my current build you can decide how to build yours:

Summoner spells:
-Flash/teleport (occassionally I use ignite, fortify, cleanse)

Masteries:
-0/27/3

Runes:
-MP5 FLAT Quints/Yellow. Red Magic pen, blue Magic resist (per level or flat)

Items:
- Regrowth pendant
- Philosopher stone
- Boots
- Heart of gold (only if you are doing poorly in your farming or if you KNOW you will get randuims).
- Lucidity Boots

Here things change a lot but I usually get:
- Sunfire OR 2x Negatron Cloak & 2x Chainmail (only 1 chainmails if got heart of gold.)
- Since both negatron cloaks and chainmails upgrade to the most powerful defensive items I just upgrade them depending on what I need the most. Sunfire, Thornmail, Frozen hearth, Randuims for armor. Force of nature, Banshee's, Abyssal Scepter, QSS for Magic Resist.

Personally I always feel squishy if I'm using warmoggs, always. But lets think of it for a minute:

- Mundo's Cleaver do a % of your max HP
- Warwick's Q does the same
- Malzahar poll does the same
- Xin does the same (current HP)
- Blood razor does the same
- Deathfire grasp does the same
- Kogmaw's W does the same
- Amumu's despair does the same
- Garen's Ulti does damage based on your missing HP...
- Nasus's ulti also does it.

If you receive 3 spells for 300 damage each you are down 900 HP. Even with 50 magic resist you get hit for 600 Damage.

My point is... warmoggs is a highly situational Item... and in almost every case other item would be better for survival... notice the ALMOST part

- VS heavy magic: FoN, Banshee, Guardian Angel, Boots, Aegis, Abyssal
- VS Heavy Physical: Thornmail, Randuims, Zhonya, Guardian Angel, Frozen Hearth, Boots

Another way to put it:
- Warmoggs costs 3000 gold.... with 3000 gold you can get 2 negatron cloaks and 2 chainmails. Sooo 96 Magic resist and 90 Armor. If you already have 2000 HP... it is like adding an aditional 2000 HP.

Is it a completely useless item? no, warmoggs has its uses, it is just that there are better items. Items that give you AP for more damage, Items that give you CDR for a 36 second feast. items that give you Auras to protect your allies or increase their damage. There are just better items.

When would I use Warmoggs?
- If I have to fight 2-3 very heavy true damage champions like Irelia, Olaf, Corki and Chogath. OR if I'm planning to use Atma's because we completely lack of Physical damage champions.

I personally like to open Regrowth into Philosopher. Helps him lane a lot. Then I usually work towards an Aegis if we really need me to tank. RoA could also work in that spot if the tank spot is covered. I generally like 3 speed boots, but merc is easily next choice. Then just build survival AP items like Abyssal and Rylai. I feel Cho is best as a beefy caster. He cannot really full tank effectively, as you are generally ignored. He also is not greatest initiator, but he is great after following up an initiation, as a well placed Rupture + Scream = Win

If you are doing really, really well, and feel you will be able to farm like a super beast, and rack a ton of kills, you can grab a mejai early for a more potent AP build. With feast you should have no issue getting kills. And since you have a ton of gold, your full set would look something like Rylai, Abyssal, GA (Could be replaced with Hourglass if you are THAT good), Rabbadan, Mejai, and boots. That build doesn't get to happen that much, but it is so fun when it does. 4K hp tanks with at least 500 something AP is beyond scary. Even more fun with glowing elixir power!