Oak's Parcel is a key item to give Professor Oak in Pallet Town. Upon entering the Viridian City Poké Mart, the clerk will ask you to hand it to Oak. In Pokémon FireRed and LeafGreen, Professor Oak mentions about that there was a custom Poké Ball in it that he ordered.
Locations[]
R/B/Y/FR/LG: Viridian City Poké Mart