The Collaborative International Dictionary of English v.0.48:

Oleaceous \O`le*a"ceous\, a. [L. ol['e]aceus of the olive tree.]
   (Bot.)
   Of, pertaining to, or resembling, a natural family of plants
   (Oleaceae), mostly trees and shrubs, of which the olive is
   the type. It includes also the ash, the lilac, the true
   jasmine, and fringe tree.
   [1913 Webster]

WordNet (r) 3.0 (2006):

oleaceous
    adj 1: of or pertaining to or characteristic of trees or shrubs
           of the olive family