An over-the-top golf swing is one of the most common types of swing faults. When you have an over-the-top swing, there are generally two results you can expect at impact: a slice or a pull. It's all determined by the angle of your club face at impact. Because the problem is so common, there are a variety of drills that can be used to cure an over-the-top swing.
Lead Arm Tight
One of the common symptoms of an over-the-top golf swing is a lead arm (the arm closest to the hole) that stretches away from the body as you go into the downswing. Take some controlled swings and you should notice this if you have a tendency to swing over the top. A simple drill can help correct this. When practicing, keep your lead arm tight to your body throughout the backswing and the downswing. After some time you'll start to get the feel for keeping your club in tight.
Over-the-top golf swings bring the club to the ball from outside the swing plane. An excellent way to fix this is to focus on keeping your club head inside to square, then back inside during the course of your swing. You can use a drill to practice keeping your club inside. Take a golf club head cover and place it an inch to the outside of your ball. You can start off a little farther away (say 2 inches) if it makes you more comfortable. Practice swinging so that you don't touch the club head cover when you come out of your down swing through impact. As you become more proficient, move the club head cover closer to reinforce an inside to square to inside swing path.
Another way to fix an over-the-top golf swing works through recognizing rotation flaws in the swing. When you swing over the top, you tend to bring the club in shallow, your body rotating to compensate for the inside move. But, the body overcompensates pushing your club head out. To solve this, hold a golf ball in the hand of your non-lead arm (arm farthest from the target). Bring that hand up as if you were swinging a club and then back down, throwing the ball underhanded as you come through the impact zone. You'll want the ball to fly at a right angle away from you. If the ball is flying out straight, or to the left, you are coming over the top and across your body. As you practice throwing the ball out to the right, you'll learn the feel of proper rotation and arm movement needed to fix the problem.