I know you will find another Oran Berry later in the game; I skipped the Miltank thing (it's not important for completing the game) and eventually came across another berry, though I don't remember exactly where.
You can get Blue Shards from smashing rocks, then trade with the Juggler in Violet City.
But probably the easiest way is to 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.
Once you have one Oran Berry, transfer it back to your game, plant it in your Berry Pot, and water it regularly (about every 4 hours).
You should get several more, which you can replant again and again.
One more option: on Fridays, the Daily drawing corner has oran berries as a 3rd place prize.
Costs 300 per drawing, can be done as many times as you have the money for it and is an easy way to rack up oran berries for the moomoo farm ahead of time.
gaming.stackexchange.com/users/1298/DisgruntledGoat
There are four ways to acquire Oran berries: 1.
Catch a Pokémon which carries Oran berries.
In HeartGold/SoulSilver, the following Pokémon have a 50% chance to carry an Oran Berry when encountered.
Capture them or use a Pokémon with the move Thief or Covet (while carrying no item) to take the berry:
Pikachu (Viridian Forest, rare) Furret (Route 1, rare) Linoone (Many routes, while listening to Hoenn Radio)
The following Pokémon have a 5% chance to carry the Oran Berry: Sentret (Route 29, Route 1) Zigzagoon
(In towers, while listening to Hoenn Radio) Bibarel, Pichu, and Raichu also carry the berry, but cannot be caught anywhere in HeartGold/SoulSilver.
2.
Get the Berry Pots item from the flower shop in Goldenrod City.
The Berry Pots is a key item which allows you to plant berries and grow new ones.
Water them every four hours or more for best yield.
The Flower Shop is easy to miss, as Goldenrod is a big city.
Search all the back-streets until you find it.
3.
Use the Pokéwalker.
If you unlocked the Yellow Forest route during the event, Pikachu caught on this route often carry Oran berries.
Otherwise, they are commonly found as items on the Refreshing Field route.
No other routes have the Oran berry.
4.
Give blue shards to Juggler in Violet City.
Shards are acquired by using Rock Smash on rocks.
The Juggler in Violet City will give you one Oran berry for each blue shard.
The Pokéwalker routes Night Sky's Edge, Scary Cave and Warm Beach also carry blue shards as rare items.
5.
Trade from another game.
Oran berries are easily grown in Pokémon Diamond/Platinum, and appear on wild Bibarel.
Trading online for any of the Pokémon listed above as carrying Oran berries might show up some carrying the berry.
Additionally, shards were easier to come by in those games.
gaming.stackexchange.com/users/3454/Joe Dovahkiin
It's pretty difficult to find specific berries in HeartGold and SoulSilver.
Take as many Oran Berries as you can (if you have none, exchange a Blue Shard to the Juggler in the Violet City Pokemon Centre) and plant them in your Berry Pot.
Keep planting until you have enough (which I believe is 10 berries ).
gaming.stackexchange.com/users/18/Kevin Yap
There is an Oran Berry at the Ruins of Alph at the room with the Kabuto Puzzle.
When you face the plaque at the end of the room, use the escape rope.
That will open the plaque and you can enter the other room which contains four items.
They are: Energy Powder, Heal Powder, Oran Berry, and Pecha Berry.
Just plant the Oran Berry in a berry pot.
gaming.stackexchange.com/users/51324/user51324
Using pokewalker, Blue shards, Mom buying you stuff etc.
gaming.stackexchange.com/users/66586/user66586
You receive Oran berries when you receive the berry pots after fighting Sudowoodo.
You can harvest more by using the berry pots.
One thing to keep in mind for the future is to never use your last berry.
Always farm more in the berry pots first.
gaming.stackexchange.com/users/1719/Invader Skoodge
An easy way to get one if you didn't do it already is going to the ruins of alph, there is a puzzle to be solved in a ruin right above the research center.
If you done the puzzle go to the wall behind it where there is something on the wall.
If you stand in front of it and use an escape rope you can get to a secret room.
There you'll find 4 pokeballs.
One of them contains an oran berry.
Then grow some more and you're done!
gaming.stackexchange.com/users/106550/marianne
How do I get more Oran Berries in Pokemon Heart Gold or Soul Silver to cure the Miltank above Olivine City from its illness?  I did not create more Oran berries in my berry pots and have run out of my initial batch.  Is there a Pokemon that might be holding it if I catch it?
How do I get more Oran berries in Pokemon Heart Gold or Soul Silver?
gaming.stackexchange.com
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gaming.stackexchange.com/questions/5500
gaming.stackexchange.com/users/660/TeeOh
Go to the flower shop in Goldrenrod city, Violet City, the refreshing field or the ruins of Alph.  Therea are multiple ways to get them, including sending a pokemon to the refreshing field area, getting the berry pots item, giving blue shards to Juggler in Violet City, fighting Sudowoodo, going to the ruins of alph or simply cathcing a pokemon with one on it.
Users suggest multiple ways to get oran berries.