How do you setup external collision? For example a medicine ball hitting the character's belly.

The docs only say:

Unlike Vellum, there are only two inputs to the solver, one for the Geometry and one for Constraints. Currently, any collision geometry must be pinned simulation geometry.

Specify a group for your external collider, then merge it with the bone input. After your Otis Configure, put down a point wrangle that runs over that group with the following snippet:

Furthermore, your collider must have a `tpose` attribute, and be triangulated.

On your Otis Configure node you'll want to make sure that the Bone Group field excludes the collider, so that it doesn't have any constraints created with it, although if your collider is far away on the start frame then that probably won't happen anyway.

I'll look into updating this on the docs.