
You don't lose unspent green mana as steps and phases end. Omnath gets +1/+1 for each unspent green mana you have.
How do you want this deck to play?
What the commander does best.
Bank a mountain of green mana, watch Omnath balloon, then trample over someone's face with him.
231 cards fit this theme
Omnath leads a tribe of elementals whose enter-the-battlefield triggers pile up value while your mana stays banked.
110 cards fit this theme
Untap your mana dorks again and again, then dump all that floating green into one enormous Omnath.
111 cards fit this theme
Off the beaten path, but they work.
Every land drop adds another green mana to the pile, growing Omnath and firing off landfall triggers.
438 cards fit this theme
Use the hoarded mana on X-cost monsters, growing +1/+1 counter creatures and multiplying them.
793 cards fit this theme
Only you keep your mana between phases, so you tax and lock the table while Omnath quietly grows.
50 cards fit this theme