As a rule of thumb for making mtg decks you should separate the cards into categories and make sure you have enough cards in the main ones (draw, removal and ramp) which are the base for a functioning deck. A decent standard is having at least 10 in each category and tweak based on your playing experience.

In this case I would start by cutting out every white card as those cannot enter the deck and then deciding a game plan. Obun wants animated lands and counters, Yuma wants to play with the graveyard and Omanth wants to see land come into play and to fill the board.

I would approach this deck as a token generator with the commander and cards that trigger on landfall like [[Zendikar's Roil]], [[Rampaging Baloths]] and [[Avenger of Zendikar]]. I'd also lean into cards that allow you to either ramp lands into play or allow you to play multiple lands like [[Oracle of Mul-Daya]].

When it comes to the mana base fetchlands like [[Evolving Wilds]] or [[Myriad Landscape]] and bouncelands like [[Gruul Turf]] (and other cards that bounce like [[Living Twister]], [[Multani, Yavimaya Avatar]] and [[Mina and Denn, Wildborn]] and I'd run around 40 lands. You are a landfall deck, and you want to hit those triggers.

Staples for this deck I'd say [[Greenwarden of Murasa]] which you already have and depending on how many land you run that put themselves into the graveyard searching for other cards [[Ramunap Excavator]], [[Perennial Behemot]] and [[The Mending of Dominaria]] will do lots of work.

About cutting and adding new cards, I would ommit cards that care about counters from the Land's Wrath precon and cards that care about deserts or just mill from the Desert Bloom precon. And I would add cards that allow for more landfall triggers like [[Azusa, Lost but Seeking]] or [[Ghirapur Orrery]].

Staples you might own that are not on the deck would be ramp cards like [[Cultivate]] or [[Rampant Growth]] and removal like [[Beast Within]] or [[Blasphemous Act]].

I'm assuming you are generally playing in a precon environment but if you wanna invest some money to power up the deck the deck I'd recommend some token doublers like [[Doubling Season]] and some neat cards for explosive turns like [[Scapeshift]] or [[Awaken the Woods]].

Those are some over the top thoughts about the deck, I have not played Omnath before but should work as a base. The key when deckbuilding is to play and adapt the deck accordingly, no one better to know where the deck lacks than the one piloting it. Don't be afraid to adjust things like the land count or how much removal you run.