First of all, you get the 6 mana investment of a card each turn
Oko can't just win the game on the spot and does have a far weaker presence in Commander due to the nature of Planeswalkers being more vulnerable in a multiplayer format

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The Ninja card stays revealed and isn't put onto the battlefield until the ability resolves
Sometimes, the best-laid plans go awry, your hand is empty, and you want to have something on the board that can accrue value for you by simply turning your lands sideways
In addition to the great artwork, this card comes with an exciting new token-related mechanic