The point is to get as high of armor and magic resist as possible with health regen to back it up. If you get low or survive the initial burst, you only need to get away for a brief period before you have healed enough to change the dynamics of the fight. It is almost impossible to get more than 200 damage from any single magic ability or physical attack against a Malphite using this build, so the survivability is really good overall.
Omen just gives low armor for the purpose of this build, so despite it's passive/active, its cooldown reduction, its health and health regen, it does not provide enough armor to really keep you up. The closer you get to 400 armor or more, the less physical damage characters do to you, it practically voids the effects of last whisper, black cleaver, etc. Besides, the health benefit from Randuin's Omen is an unnecessary waste of gold. With 115 or more health regen in game (this system doesn't factor in health regen from mana) each second you survive is effectively adding 23 health to your total health pool. The damage reduction from this build is so high, that only the strongest nukes on characters with tons of penetration will actually do more than 200 damage at a time. You survive most bursts and heal most of the health back during the gaps in the bursts, so maximum health ultimately doesn't matter, and will even increase the damage you take from some abilities. To be honest, after playing my own build in a few games, I found you do not need the warmog's, I should probably edit it out and replace it with another force of nature (puts the total magic resistance of the build up to 300 at level 18).

Lol, prefer it over an Omen, u mad. I'd prefer it over a Thornmail, because it's a more reliable damage dealer. But over an Omen? Lol.

I would consider a sunfire cloak over a Omen or a second Frozen heart. The damaging passive plus bonus hp plus the armor would make it a great addition.

If you choose against that idea, I would go with an offensive aura to help your dps. A Starks, an Abyssal Scepter, or a WotA.

If you choose against this even, you should at least get a GA or a Aegis. Basically any option is better than a second Frozen Heart.

I don't get the reasons you give for not getting a Randuin's Omen. The item would be a PERFECT addition to this item build (forget about the second Frozen Heart, the passive is unique, so it's wasted money). A Randuin's is perfect because of both its passive and its active.
Passive: reduces attack speed by another 25% when hit (on chance), so you take even less damage.
Active: slows nearby enemies' AS and MS, which can give the enemy team a huge positional disadvantage. But the best thing about this item is its tanking role: the CC duration increases by 0.5 second per 100 resistance (armor%magic res). With this item build and Brutal Strikes, you can get your total resistance to over 600 (or 700?), which ends up in a 6 or 7 second AoE slow. Now that is some massive CC.

Think about it man, Randuin's is awesome for this build. I'd get it after the FoN.

I like it it is very in depth. i myself hav tanked for a while now i and i look foward to trying ur way off tanking but i do hav a kestion why wait until lvl 7 to get ground slam