Biomes O' Plenty is a mod which modifies the world generation and adds several new biomes to it.
First you have to switch your server to the forge server software (https://aternos.org/software/forge/).
In order to generate a Biomes O' Plenty world you have to install the mod on your server: https://aternos.org/addons/a/curseforge/biomes-o-plenty
MINECRAFT 1.12.2 and older: Type "BIOMESOP" into the worldtype field.
MINECRAFT 1.13.2 and newer: Type "biomesoplenty" into the worldtype field.
Complete the process by clicking the generate button. Please note that your current world will be deleted.
As last step you only need to start your server.
Please note that you need to install forge and the Biomes O' Plenty mod on the client too.