If the target creature is an illegal target as Oko's first ability tries to resolve, it won't resolve and none of its effects will happen. Oko won't gain hexproof.

Changing the target or targets of a spell or ability won't affect whether or not the controller of that spell or ability has committed a crime. Only the initial targets chosen for that spell or ability are used to determine whether or not its controller committed a crime.

While Oko is a copy of a creature, he isn't a planeswalker. He won't have his loyalty abilities, and damage dealt to him won't cause him to lose loyalty counters.

If the copied permanent is a token, the token that's created copies the original characteristics of that token as stated by the effect that created that token.

A player commits a crime as they cast a spell, activate an ability, or put a triggered ability on the stack that targets at least one opponent, at least one permanent, spell, or ability an opponent controls, and/or at least one card in an opponent's graveyard.

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

A player can commit only one crime per spell or ability they control. Targeting multiple opponents, permanents, spells, abilities, and/or cards with the same spell or ability doesn't constitute committing multiple crimes.

If the copied creature is a token, Oko copies the original characteristics of that token as stated by the effect that created that token, with the stated exception.

Except for the listed exception, Oko copies exactly what was printed on the target creature and nothing else (unless that creature is copying something else or is a token; see below). He doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on.

When Oko becomes a copy of the target creature, he's neither entering nor leaving the battlefield. Any enters-the-battlefield or leaves-the-battlefield abilities won't trigger.

Each of the tokens created by Oko's last ability copies exactly what was printed on the original permanent and nothing else (unless that permanent is copying something else or is a token; see below). It doesn't copy whether that permanent is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on.

Any enters-the-battlefield abilities of the copied permanent will trigger when the token enters the battlefield. Any "as [this permanent] enters the battlefield" or "[this permanent] enters the battlefield with" abilities of the copied permanent will also work.

If the copied permanent is copying something else, then the token enters the battlefield as whatever that permanent copied.

If the copied permanent has {X} in its mana cost, X is 0.

The spell or ability that constituted a crime doesn't have to have resolved yet or at all. As soon as you're finished casting the spell, activating the ability, or putting the triggered ability on the stack, you've committed a crime.

For example, an ability that triggers when you cast a spell that targets an opponent will trigger at the same time as an ability that triggers whenever you commit a crime. Those abilities can be put on the stack in either order (if you control them both), and they'll both resolve before the spell that caused them to trigger.