Possibly taken from the plant family, Nerium oleander (flowering shrub known as oleanders), Cascabela thevetia (yellow oleander), Acacia neriifolia (oleander wattle); or a species of moth, Daphnis nerii (oleander hawk-moth).
In the complex language of love practiced during the time of Queen Victoria, the Oleander flower means caution.
A diminutive use of Oleander could be Ollie, Lee, Lee-Ann, or Anders.
As an English name it can also derive from the month of August, which was named for the Roman emperor Augustus.