Chansey uses it. Other people said Happiny uses it, but their wrong.
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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.
The Oval Stone is an item in the Pokemon games which can be used to evolve certain Pokemon. For example, it can evolve Chansey.
You can evolve happiny with an oval stone after its friendship is high
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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.