One needs to increase Chansey's happiness to its maximum.
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Give it to your Happiny at day time and level it up to evolve it into a Chansey.
you cant... you have to make chansey evolve with high friendship. you can find chansy on rout 209 in diamond, pearl, and platinum
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 does not evolve by level. it evolves by happiness. i hope this helped!
Chansey > level up with max happiness > Blissey