Oran berries are given by a Juggler in Violet City in exchange for a Blue Shard
If you already have one you can grow some more in the berry pot, and can use Gooey Mulch to increase the production or Oran berries! :D
Also, assuming you got the DS version and Pokewalker, you can send a Pokemon to the Refreshing Field area on the Pokewalker. Once you have 300 steps in one day, use the Dowsing Machine and there is a good chance you will obtain an Oran Berry.