After you defeat or capture the Sudowoodo on Route 36, A man will say he was entertained by the battle and give you a few berries, on of them being Oran, he will also give you an item that you plant berries in it to make more berries. There is also a man in Violet City by the pokemon center that will give you berries for Shards, I believe that if you give him a Red one, he will give you a Oran Berry among other types of berries.