Assuming oversleeping isn't a chronic issue, the fatigue you experience after sleeping in likely has to do with the disruption of your sleep cycle. Internal and external factors work to set your sleep cycle (or circadian rhythm), and when you break that rhythm, you're left feeling tired and out of whack.
You should also consider your recent sleep history—if you're in "sleep debt," a one-night sleep explosion to get you square with the sleep bank won't work. To return to well-restedness, it's best to add an hour or two of sleep per night, reports the Scientific American.
If you regularly experience fatigue, however, you should consult a doctor, as this could indicate larger health issues. Conditions from anxiety to heart disease could be at play; lifestyle factors like substance abuse and unhealthy eating habits can also cause fatigue.