EDH rules say that the only mana that can be in your mana pool is that which matches your general's color identity; if you could produce something your general is not (Birds of Paradise ) or if your general changes color (Nim Deathmantle ) all mana that is not of your general's identity becomes colorless as a statebassed effect. And since Omnath's size is dependant on your green mana pool, if he becomes black your green mana pool is now empty, thus omnath is now only a 1/1

You cannot change your generals color identity... this is the reason Celestial Dawn locks out decks that don't have white in them when you donate it.

I am with Victrix on this. You commander's color identity is determined when you construct your deck, and can not be changed during the course of the game. For normal game mechanics, of course, it can be altered. But the commander color identity never changes, regardless of what happens during the game. You can still only produce (and will always be able to produce) the type of mana available to you based on the actual color identity of the commander you are playing. Naturally this excludes specific circumstances that prevent you from producing colored mana because of various in-game mechanics, but the in-game restrictions and alterations never override the game format rules.

I stand corrected... Rules lawyer at my shop got all over my case about that combo so I took it out... Looks like it should go back it.....

No worries man. The same thing happens to me. People seem so sure of themselves that it makes it hard to know right from wrong sometimes.

Sylvan Library seems good.

No advice, just love. This deck makes me fall in love with green all over again <3

Rancor that is all. Add it to commander and it keeps coming back if he dies.

Spearbreaker Behemoth with the addition of a hexproof ability like Asceticism to make Omnath almost unkillable because you will always have the mana to sustain him

Have you thought of Hydra Broodmaster to pump out a lot of tokens? IDK if you need it as an essential but it seems to fit in with the theme. You also should probably put Helix Pinnacle in the main deck. With this amount of ramp you might not even need Omnath to pull off 100 mana. Karametra's Acolyte + Staff of Domination= infinite mana with 5 green devotion. Coolest mono-green I've seen in a while.

A couple game stalling creatures I run in mine are Platinum Angel and Platinum Emperion. They won't win you the game but I have survived long enough to defeat my opponent by having them on the field. Worth considering anyways? Also, artisan of kozilek!

Staff of Nin and Mind's Eye are good ways to get extra draws without potentially screwing yourself after casting primal surge.

A good backup plan in case of stupid tricks such as Cyclonic Rift would be to have some kind of sac outlet like Spawning Pit or Altar of Dementia. If they try to blow up your board state somehow, you can sac a large number of creatures and then sac Ulamog to give you back some kind of usable library for the next turn. This can also protect you from being forced to draw cards.

Shaman of the Forgotten Ways would be a good addition to this build.

I actually just ordered a Shaman of the Forgotten Ways recently, I'm looking forward to trying that out

This may be a awful suggestion, but ya I am going to try even though I am like clueless with green, how about Nissa, Worldwaker even though it wouldnt surprise me if youve considered her before and I am just late XD very interesting deck I like it!