From StrategyWiki, the free strategy guide and walkthrough wiki

[edit] Professor Oak's Aide

Inside the Guard Post building on Route 15, east of Fuschia city, you will find one of Oak's Aides upstairs. Talk to him, and if you have complete data on 50 types of Pokémon, he will reward you with an EXP. Share. Giving this item to a Pokémon allows it to receive some of the experience from every battle without bringing it out.