OnMouseOver is called every frame while the mouse is over the GUIElement or Collider.

	// Fades the red component of the material to zero 
	// while the mouse is over the mesh
	function OnMouseOver () {
		renderer.material.color -= Color(0.1, 0, 0) * Time.deltaTime;
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void OnMouseOver() {
        renderer.material.color -= new Color(0.1F, 0, 0) * Time.deltaTime;
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def OnMouseOver() as void:
		renderer.material.color -= (Color(0.1F, 0, 0) * Time.deltaTime)

This function is not called on objects that belong to Ignore Raycast layer.

OnMouseOver can be a co-routine, simply use the yield statement in the function. This event is sent to all scripts attached to the Collider or GUIElement.