Does Ob Nixilis, the Adversary's casualty cost let you sacrifice a token to copy it?

Ob Nixilis, the Adversary has Casualty 3, meaning as you cast it you may sacrifice a creature with power 3 or greater to copy the spell and choose new targets for the copy. The Casualty ability (introduced in Streets of New Capenna) does not restrict what kind of creature you sacrifice — tokens are creatures and are entirely legal sacrifice targets as long as they meet the power requirement.

The relevant rules base is CR 702.157 (Casualty). It functions as an additional cost paid during casting (CR 601.2b). Any creature you control with power 3 or greater qualifies, including creature tokens such as a 3/3 Beast token or a copy of a creature made by another effect.

The copy of Ob Nixilis, the Adversary is created on the stack (CR 706.10) and resolves as a separate spell. It will attempt to enter the battlefield as another legendary Ob Nixilis, triggering the planeswalker legend rule if you already control one (CR 704.5j). You can keep whichever one you prefer and put the other into the graveyard.

Concrete example: You control a 3/3 Centaur Warrior token. You cast Ob Nixilis, the Adversary and pay the Casualty 3 cost by sacrificing that token. A copy of Ob Nixilis, the Adversary is put onto the stack. Both copies resolve (assuming no responses), and you get two Ob Nixilis planeswalkers entering — though the legend rule immediately forces you to keep only one.

Bottom line: tokens are perfectly valid for Casualty costs. The only requirement is that the sacrificed creature's power meets or exceeds the Casualty number at the time of sacrifice.