My little venture into mono-green commander. Mono coloured means a nice cheap mana base. The idea here is, as with any Omnath deck, to ramp as hard as possible and then swing for lethal commander damage.
To support the walking ball of mana, I've gone for an enchantment sub-theme. This includes a number of "enchantress" effects - Argothian Enchantress, Verduran Enchantress, Eidolon of Blossoms and Enchantress's Presence all aid in generating card advantage, which is important for a mono green deck. Cards like Vernal Bloom and Mana Reflection are obvious choices that both pump the mana pool quickly, and trigger any enchantress effects on the field. A major part of the enchantment base is protection, in the form of the various umbra cards for one-time protection from destruction, to stronger cards like Asceticism that make the opponent hate you.
For more interesting utility cards, we've got Defense of the Heart to provide us free enchantresses, trample from Nylea, God of the Hunt or Thunderfoot Baloth, or even turn our enchantress creatures into gigantic deadly monsters with the amazing Pathbreaker Ibex. Simply having Defense on the field is a fantastic method of forcing your opponent to sit still and let you build up board presence, lest they let you slam some free creatures onto the board. As well as this, we have the interesting Broken Fall. While the effect is nice, the really interesting part of this card is the cost of this effect. Late game when our mana pool is hitting the triple digits, being able to repeatedly return and re-cast this card with one or two enchantress effects on the field provides amazing card advantage for a relatively low cost, a huge boon for our late game.
Another late game mana-sink is Helix Pinnacle, an obvious alternate win condition. Having shroud makes it very very difficult to remove, and generating 100 mana isn't that hard with just one mana doubling effect and things like Seedborn Muse supporting us. While playing this card on turn one is tempting, it puts a big target on your head so it's best to save it for later when you can play it and immediately put all 100 tower counters on it - a great strategy against pillow fort players.
Should the opponent remove your threats and important enchantments, we have Regrowth, Praetor's Counsel and Eternal Witness which let us grab cards from the graveyard for easy re-use.
Ground Seal acts as a nice cheap cantrip and stops reanimator decks dead in their tracks. While this has some anti-synergy with Regrowth or Scavenging Ooze, it's not too hard for us to make do without these effects our just destroy the card with our own Seal of Primordium. For one of the coolest ways of abusing Omnath's ability to get really huge, we have Greater Good. If you can moderate your mana pool by dumping mana into Helix Pinnacle or repeated abuse of Broken Fall's effect, you can draw out your entire deck to look for that last card that will push you to victory by sacrificing Omnath, then recasting him by using just a small fraction of your huge mana pool. Just be careful - if Omnath's power is higher than the number of cards left in your library, you will instantly deck yourself out and lose the game. The best part of this card though is that its effect is repeatable should the opponent not remove it, a key factor in the utility of the card.
Being able to evade opposing creatures is key for what is essentially a voltron deck, so Rogue's Passage, Bladed Pinions and even Treetop Bracers are here to help Omnath push over whatever creatures the opponents is trying to chump you with. Sometimes you don't want to evade though, and since green is somewhat lacking in creature destruction effects, Grappling Hook is a nice card that not only grants Omnath deadly Double Strike, but allows you to systematically remove whatever annoying creature your opponent is trying to set up with. We have plenty of ways of providing Omnath with the original green evasion method, Trample, too, including Cartouche of Strength and Nylea, God of the Hunt.
Since we're relying on Omnath himself to be dealing almost all of our damage, it can be a pain when he gets Imprisoned in the Moon or gets hit with Pacifism, or any multitude of other irritating road blocks. As such, a big portion of our instant and sorcery base is dedicated to hate and removal, with great cards like Desert Twister, Bramblecrush, Krosan Grip Acidic Slime and Beast Within helping to deal with any kind of annoying permanent that's blocking your path to victory cough cough Paradox Engine cough cough.
Also present here are a number of great mono-green staples that an Omnath deck just couldn't do without - Eternal Witness, Regrowth, Worldly Tutor and Harmonize all help with consistency and card draw, Skyshroud Claim and its mana-ramping friends are ever present in any deck that likes large amounts of mana, Vigor provides Omnath with a way to completely ignore damage (beware, dropping this will put a HUGE target on your head), Burgeoning and Exploration provide excellent first turn plays that push our mana base forward even faster, and Tooth and Nail allows for a quick one-two-punch with whatever pair of scary creatures you'd care to fetch out.
Overall, I'm currently very happy with this list. There's plenty of redundancy in the effects we have and we certainly don't rely on a single card in the 99 to push us through to victory. This makes the deck consistent in its goal - make lots of mana, deal with whatever is stopping Omnath from hitting the opponent square in the face, then end the game swiftly. I enjoyed building around the enchantment theme as well, since a lot of Omnath lists I've seen on this site seem to end up being 'mono-green goodstuff', which isn't something I find very interesting to build or play.
Hopefully you like the list and will look into playing a little Omnath, Locus of Mana yourself. He's a lot of fun and relatively cheap to build since the mana base is so simple. Have fun!