How to reference another object's collider for OnCollisionEnter.

Just as the title says. I assume you have to use GetComponent.

That collision event has the following form: Collider.OnCollisionEnter(Collision other). This means the collider receiving the event also receives an object of type Collision attached to the event as a parameter, on of the properties in that class is the collider property which contains the collider that was hit. Basically you just need other.collider to access it.

You can also get and use any components from the hit object using:

T component = other.gameObject.GetComponent<T> ();
if (component != null) {
    component.Foo ();
}