oncollisionenter is declared but never used
//The problem is that you need to move this out of a void [e.g. void Start() or void Update()]
void OnCollisionEnter(Collision collision)
{
if (other.gameObject.tag == "Object")
{
Debug.Log ("Collided");
}
}
//Also same with OnCollisionEnter2D, OnTriggerEnter and OnTriggerEnter2d