Do you know anything else about this name?
Are you sure it's not a variant of Oleander? If it is, here the meaning:
http://etymonline.com/index.php?allowed_in_frame=0&search=oleander&searchmode=none
olea means "olive tree", but that doesn't work for Oleanthe since it'd combine a Greek and Latin element.
In my Ancient Greek dictionary it says pretty much all words starting with
Ole- in Greek mean something along the lines of "destruction", "disastrous" or "kill"
I did come across
olunthos which means "unripe fig", "wild fig tree" or "male flower". This at least would be a plant-like meaning which you'd expect with Oleanthe.