C# Script Not Working?

First, you don't need to define your type twice, only do that in the first part of your script. What I mean is you don't have to type 'float' in front of your variables inside of your function (void) because you already did this, so the variables are not local to the function. Also, you're only calling Oh() ONCE, in Awake(). Not sure why, you could just put all of your code inside of Awake() instead of using a separate method. But if you're trying to check for something all the time (every frame), you need to use Update(). Your question is vague, so I'll just throw your if() statements into Update() to show you what I mean. Try this, see how it goes, then get back at us with a better description of your issue.

 using UnityEngine;
 using System.Collections;
 
 public class EnemyAI : MonoBehaviour
 {
     //wall Boundaries
     float wall_left;
     float wall_right;
     float wall_top;
     float wall_bottom;
     
     //AI properties
     Vector2 AI_Position;
     float Speed;
     
     float randomX;
     float randomY;
     
     Vector2 randomXY;
     Vector2 Direction;
     
     private void Awake()
     {
         //Stuff you do on Awake.
         Oh();
     }
     
     private void Oh ()
     {
         Speed = 0.3f;
         wall_left = 5.0f;
         wall_right = Screen.width - 5.0f;
         wall_top = Screen.height - 5.0f;
         wall_bottom = 5.0f;
         
         //Get two Random values within a Range (Screen dimensions)
         randomX = Random.Range(0,Screen.width);
         randomY = Random.Range(0,Screen.height);
         
         //Create a Vector2 out of the Random values
         randomXY = new Vector2(randomX,randomY);
         
         //Get the Direction from AI to the RandomXY generated
         Direction = randomXY - AI_Position;
         
         //Normalize the Direction to apply appropriatly
         Direction = Direction.normalized;
     }
 
     void Update()
     {
         //Check that your AI is within your boundaries
         if( AI_Position.x > wall_left && AI_Position.x < wall_right
            && AI_Position.y > wall_bottom && AI_Position.y < wall_top )
         {
             //Make AI move in the Direction (adjust speed to your needs)
             AI_Position += Direction * Speed;
         }
         else
         {
             print ("Out of bounds");
             //make your AI do something when its not within the boundaries
             //maybe generate a new direction?
         }
     }
     
     
 }