Oval stones are one of the evolutionary stones that you can find in both Pokemon Brilliant Diamond and Shining Pearl. These stones, like others, evolve specific Pokemon when given to them. The Oval Stone is used to evolve Happiny into Chansey. You have to make Happiny hold the oval stone and level it up during the day for it to evolve. This is where you can find the oval stones.
There are two ways to get the oval stones during the game. The first method is to go to Route 209 which is south of Solaceon Town. After traveling south, you will see the tower on the right. Enter the tower and go to the second floor. You will find the Oval Stone in front of the Youngster manager.
The other method of obtaining the oval stones requires some farming. Go to the Trophy Garden. You will be able to find Happiny in the wild here. Whenever you encounter a wild Happiny, they will have a small chance that they already have an Oval Stone in their hand. Capture it to get the oval stone it has on it or use a move to steal items like Thief.