Mint and oregano are highly sought after in Red Dead Online as they are often the ingredients for daily challenges. Here is where you can find these herbs.
Red Dead Online usually spawns mint and oregano herbs close to rivers and creeks. One of the most consistent places of finding these is just south of Valentine, at Caliban's Rest.
The location is pretty straightforward, just fast travel to Valentine and then proceed south along the river. At the point where it bends, you will find a high concentration of both Wild Mint and Oregano plants. You can check the image below for the exact location. The red X marks the location of the river bank so make sure you cross the road before looking for the plants.
Keep in mind that both of these plants are often next to the river banks and not exactly at the spot where the water and land meet. As you can see on the header image, there is the entire road between my character and the river itself and he is standing between three Wild Mint nodes.
When you get to the point where my character is on the map, just follow alongside the road to the south and you will come into countless Wild Mint and oregano herbs.
If you don't find any, it's possible someone picked them before you. Just join a new session and they should respawn, ready for you to harvest them.