Answer;

I have no idea.  There are a LOT more than five miracles in the Old Testament.  If you are interested, I have published a book on miracles which includes a chapter on Old Testament miracles.  It is titled  "That You May Believe." It is available at www.ipibooks.com.  

In order to be able to make a good guess, I would need the context.  Where did you find mentioned
"the five miracles in the Old Testament?"  Perhaps this is a top five list, like they have the seven wonders of the world.  In that case, it would obviously be subjective.  If I HAD to list the five most significant miracles in the Old Testament, it would be hard, but the ones which come to mind are Creation (Genesis 1), the flood, the plagues in Egypt, the crossing of the Red Sea, and the destruction of Jericho.

Honorable mention to the crossing of the Jordan, the destruction of the army of Sennacherib.   

John Oakes