As requested by Espen204 here's an updated map with Oleander Sage locations if you guys/gals would like any other additions please feel free to send me a message. Happy trails y'all! :
Where to Find Oleander Sage in Red Dead Redemption 2 - Gamepur
Guaranteed spawn of oleander (used to make poison weapons) : r/reddeadredemption
Plants & Herbs Location Guide - RDR2.org
Plants - Red Dead Redemption 2 Walkthrough & Guide - GameFAQs
Plants & Herbs Location Guide - RDR2.org
Flora of America | 12 Collectible Cards Locations - Red Dead Redemption 2 - Playthrough Guides