According to tales first recorded in Old French in the 12th century, Yseut or Ysolt was an Irish princess betrothed to King Mark of Cornwall. After accidentally drinking a love potion, she became the lover of his nephew Tristan. Their tragic story, which was set in the Arthurian world, was popular during the Middle Ages and the name became relatively common in England at that time. It was rare by the 19th century, though some interest was generated by Richard Wagner's opera Tristan und Isolde (1865).
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.
It was used as a given name for a girl in the Canadian province Alberta in 2016.