if (Application.platform == RuntimePlatform.OSXPlayer)
Debug.Log("Do something special here!");

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Awake() {
if (Application.platform == RuntimePlatform.OSXPlayer)
Debug.Log("Do something special here!");

}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Awake():
if Application.platform == RuntimePlatform.OSXPlayer:
Debug.Log('Do something special here!')