  In this paper we study those bent functions which are linear on elements of
spreads, their connections with ovals and line ovals, and we give descriptions
of their dual bent functions. In particular, we give a geometric
characterization of Niho bent functions and of their duals, we give explicit
formula for the dual bent function and present direct connections with ovals
and line ovals. We also show that bent functions which are linear on elements
of inequivalent spreads can be EA-equivalent.
