How to check adjacent cell for value and display error message for certain conditions

Hi all,

as roster coordinator for my workplace, I usually handle all the allocation of overtime when it arises. I was on vacation just recently, however, and a couple of other supervisors incorrectly allocated some overtime. To get around that human error, I've been trying to code something that would check cell values and throw up a message box if certain conditions were met. I've been trying a sheet change event using offset but without much luck so I'm hoping someone can point me in the right direction. Basically I'm trying to achieve this...

Range of cells is A4 to AA42

If a user enters a value of "L" or "OL" into the sheet, it should check the cell immediately to the left, and if it finds a value of "N" or "ON', throw up the message box. Conversely, if a user enters "N' or "ON' into a cell, it should check the cell immediately to the right for a value of "L" or "OL", and if found, throw up a message box.

Thanks in advance for any suggestions,

AJ