To get Oran Berries, Go to the betting corner at the sixth floor of the Goldenrod Department Store and pay 300 to draw a ticket that wins you a #1, #2, or #3 prize. The #1 prize is the TM of the day (the woman next to the ticket counter tells you what prize #1 and #2 are). The #2 prize is a special Pokeball, while the #3 prize is a random selection of one of ten different berries, including the Oran. Do not waste your money by trying to get a lot of them.
You must give the Miltank seven Oran Berries. The best way is to harvest the berries in the berry pots until you have seven of them. Alternatively, the girl to the left of the Miltank will give you a seal case and the girl on its right will give you three seals. The farmer woman also gives you a TM Natural Gift. Once you give the Miltank all the berries, you can buy Moomoo Milk (which heals 100 health) for 500. In Olivine, the girl in the house north of the Pokemart will give seals.