The Oval Charm is available in Pokémon Black 2 and White 2. It is obtained by catching all of the Pokémon in the Unova Pokédex (excluding event-exclusive pokémon). It increases the likelihood that the day care man will find an egg from the pokémon that you have left in day care.
What is the Oval Charm?
The Oval Charm is an item that, if you have it in your bag, increases the likelihood that the day care man will find an egg. Every time you take 255 steps while you have two compatible pokémon in day care, there is a chance that the day care man will find an egg. Having the Oval Charm in your bag improves this chance by an unknown number.
How to Get the Oval Charm
To get the Oval Charm, you have to catch pokémon number 001 through 297 in the Unova Pokédex. Those are all of the pokémon in the Unova Pokédex, excluding the following event-exclusive pokémon: Genesect, Keldeo, Meloetta, and Victini.
For Reshiram and Zekrom, as well as other version-exclusive pokémon, you can have a friend trade the pokémon to you, then you can give it back. The pokémon is considered "caught" even if it's no longer in your possession.
After you have caught all of the Unova Pokémon, talk to Professor Juniper in Nuvema. She will give you the Oval Charm.