Question

To get a Shiny Charm do you have to fill the Hoenn Pokedex or National?

Answer 1

National Dex
Shiny charm can only be obtained by completing the national dex.
The oval charm is obtained when you complete the hoenn dex and it increase the chances of finding an egg when breeding Pokemon.

Hope I helped

Answer 2

You must fill the National Dex in order to receive the shiny charm. Once the National dex is completed go to prof. Birch's lab to obtain it.

Here is a link to more info if desired

http://bulbapedia.bulbagarden.net/wiki/Shiny_Charm

Answer 3

You need to complete the National dex.

In Pokémon Omega Ruby and Alpha Sapphire, the player can recieve the
Shiny Charm from Professor Birch after obtaining all Pokémon in the
National Pokédex, with the exception of event Pokémon.

Source.