In Red Dead Redemption 2 one of the request you might get from one of your companions in your camp could be finding Oleander. You may encounter Javier sharpening his knife and he will tell you he works on poisonous weapon. In order to create it he needs Oleander and asks you to find it. It is one of the errand quests you might need for the Errand Boy trophy.
The game doesn’t show you where to find it so you are on your own. Head to the location on the map below to find it. It is located near the delta of Ringneck Creek and Kamassa River.
Not only will you find Oleander Sage there, but also a stranger walking around, looking like he’s drunk. He isn’t drunk, though. He poisoned himself when trying a fancy herb he found. This herb is indeed Oleander Sage Javier asked you for. You can press + to enter eagle eye mode and highlight the plants in yellow color.
You can help the stranger by giving him medicine if you want to. Grab some Oleander Sage from there and head back to your camp. Approach Javier and give him what he asked for in order to finish the errand mission.