Can I use Oko's ability targetting a creature with protection from green?
A creature with protection from green can't be targeted by Oko:


  702.16b A permanent or player with protection can’t be targeted by spells with the stated quality and can’t be targeted by abilities from a source with the stated quality.


It doesn't matter that his ability doesn't have a color, protection cares about the color of the source of that ability. So unless you find a Thran Lens or something similar, you won't be able to target the creature.