I think you can get it from a lady holding a umbrella in route 8 or you can find it on routes or in caves.
Happiny
The Oval Stone evolves a Happiny into Chansey however it needs to be leveling up while it's holding the Oval Stone during the day time.
Happinny is the only Pokemon that can evolve with the Oval Stone.
The only Pokemon that can be evolved with an Oval stone is a Happiny into a Chancey.
Happiny uses the Oval Stone to evolve into Chansey.
Happiny evolves into Chansey with an Oval Stone.
The Pokemon that can use oval stone in Pokemon platinum is Happiny. It is the pre evolve form of chansey. Mostly every chansy you catch have a oval stone.
Chansey evolves from an Oval Stone.
The Oval stone evolves Happiny into Chansey.
Hapiny + Oval Stone = Chancey
happini can evolve with a oval stone
The Oval Stone is an item in the Pokemon games which can be used to evolve certain Pokemon. For example, it can evolve Chansey.