If I volunteer to work extra hours, do I get paid overtime?

Anything over 40 hours a week is overtime. If a person works overtime it doesn't matter if on a volunteer basis or they were told to work it FEDERAL law requires that a person be paid time and a half. Dept of Labor.

It depends on the state what counts as overtime. Some places if you work more you get it for working on longer shift in a day. What state are you in?

Why don't you ask your supervisor if OT will be OK'd? Are you required to stay or are you choosing to stay on your own? Most employers don't/won't pay you just because you stay after quitting time and clock out later. They pay for the approved shift hours unless they make arrangements to do otherwise.

If you work enough hours to qualify for OT based on your state's laws, yes. OT isn't based on whether you volunteer or not, it's based on the number of hours you work. Generally over 40 in a work week is OT

Overtime is anything over 40 hrs in a pay week. What is your employers policy on overtime pay?