First, you must know if you are exempt or non-exempt. Before entering HR, I had the hardest time remembering the difference. Think of it this way, if you are exempt, you are exempt from overtime. So, if your position is non-exempt, then yes, you deserve over-time. If you are not sure if your position is exempt or non-exempt, ask your Manager or Friendly and/or Evil HR Rep.
Over-time is regulated by Federal and State laws. Check your state's Department of Labor website for more detailed information on your state's over-time regulations. In general, if you work more than 40 hours in a week, you get 1 1/2 times your salary for those extra hours.
I once worked for a company (again, before entering the wonderful field of HR) and I started out as a non-exempt employee. I was working on quite a few projects which would require me to stay late. After a period of time, my boss came to me and said "We are making you exempt!" I figured it's because they got tired of paying those OT dollars. Can this be done? No! This is illegal. You cannot just make a position exempt because you don't feel like paying the OT wages.
Not sure if a position should be exempt? Go here or here for good resources on determining exempt/non-exempt status.