When I charm an opponent's unit to a battlefield I control, does that unit become the attacker, or is there no attacker since it's not the opponent's turn?

Ruling: When you charm an opponent's unit to a battlefield you already control, that unit becomes the attacker because it is the one contesting the battlefield.

Nuances:

• The key factor is who controls the battlefield, not whose turn it is
• The unit contesting the battlefield is designated as the attacker