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! :
Plants & Herbs Location Guide - RDR2.org
RDR 2 | Location of the Oleander Plant / Oleander Sage | Red Dead Redemption 2 - YouTube
Sage location in Red Dead Redemption 2 | Shacknews
RDR2 Oleander Sage Online Locations for Daily Challenges (Red Dead Online) - YouTube