If the target creature is copying something else, then Oko becomes a copy of whatever that creature copied.

Oko's last ability affects only creatures you control at the time it resolves. Creatures you begin to control later in the turn won't be 10/10.

Any effects that modify a creature's power and/or toughness without setting them to a specific number or value will apply after its base power and toughness are set, regardless of the order in which those effects were created. The same is true of counters that modify its power and toughness.

While Oko is a creature, he isn't a planeswalker. Damage dealt to him won't cause him to lose loyalty.

If the target creature is an illegal target by the time Oko's second ability tries to resolve, the ability won't resolve. Damage that would be dealt to Oko won't be prevented.

If Oko becomes a creature the same turn he enters the battlefield, you can't attack with him or use any of his abilities (if he gains any).

Oko copies the printed values of the target creature. It won't copy counters on that creature or effects that have changed its power, toughness, types, color, or so on. Notably, it won't copy effects that made the target creature become a creature if it isn't normally a creature.