Download the correct version of Biomes O' Plenty to your server.
https://www.curseforge.com/minecraft/mc-mods/biomes-o-plenty

Currently Biomes O' Plenty only supports forge.

Go to your server, open the file server.properties found in the file manager tab, and edit the line level-type=DEFAULT to say level-type=BIOMESOP

Ensure to save the file.

If you already have a generated world you will need to delete your current folder to properly ensure the mod is running.