If you are eligible for overtime (that is, you don't fit into one of the exempt categories, such as for executive employees or outside salespeople), you must be paid time and a half for every extra hour you work. Because part-time employees typically work less than a full week, they usually don't earn overtime. Regardless of whether an employee is classified as part time or full time, however, the employee is entitled to overtime pay if he or she works more than 40 hours in a particular week -- or, in some states, more than eight hours in a day.
Under federal law and the laws of most states, employees who are eligible for overtime are entitled to be paid an overtime premium if they work more than 40 hours in a week. The overtime premium is 50% of the employee's regular hourly rate. For example, if an employee is paid $14 an hour, the employee would be entitled to be paid an extra $7 an hour -- or $21 total -- for every overtime hour worked.
A few states have a daily overtime standard. In these states, including California, an employee is entitled to overtime after working more than a certain number of hours a day. If a part-time employee works three ten-hour days in California, the employee wouldn't be entitled to overtime under federal law, because the employee has worked fewer than 40 hours. However, the employee would have earned six hour of overtime under California law, which requires overtime pay if an employee works more than eight hours in a day.
The law isn't concerned with how your employer classifies employees. Whether you are called a part-time employee or a full-time employee, you are entitled to overtime when you work overtime hours. Of course, part-time employees typically don't work a full work week, and so wouldn't often be entitled to overtime based on a weekly standard. If you have an exceptionally busy week, however, and you put in 50 hours, you are entitled to overtime pay. And, if you work in a state with a daily overtime standard, you are entitled to overtime if you hit the state threshold.
If your employer hasn't paid you for overtime you've earned, consider talking to a wage and hour lawyer. An experienced lawyer can quickly figure out whether your employer is complying with the applicable state, federal, and local law. If your rights have been violated, a lawyer can explain your options and help you choose the best strategy to protect yourself, including filing a complaint with the federal or state labor department or even filing a lawsuit.