How does the Overdrive button work in my Nissan Xterra?

When I start my 2002 Nissan Xterra there is a light on the dash that comes on briefly and it says OD OFF. There is a button on my shift column that is supposed to be the overdrive button. When I hit the button, nothing happens. What is the point of this button. Is there supposed to be some type of indication that the OD is working.|||The O/D Off light should come on when you push the button on your shifter. If it does not you may have a bad relay.