Ob Nixilis, the Adversary

As you cast a spell with casualty X, you choose whether to pay its casualty cost and what the value of X will be.

The copy of the spell is created on the stack, so it's not "cast." Abilities that trigger when a player casts a spell won't trigger.

Casualty N means "As an additional cost to cast this spell, you may sacrifice a creature with power N or greater." and "When you cast this spell, if a casualty cost was paid for it, copy it. If the spell has any targets, you may choose new targets for the copy."