CSHARP

gameobject on click unity

private void OnMouseOver() {
        Debug.Log("Hovering over GameObject");
    }

unity button onclick

GetComponent<Button>().onClick.AddListener(()=>print("clicked"));

unity onclick object

using UnityEngine;
using System.Collections;

public class Click : MonoBehaviour {

    public GameObject gameObjectToClick;
    void OnClick(){
         gameObjectToClick.SpecialFunction(); //call special function you wanted to call
    }
}