Osserman and conformally Osserman manifolds with warped and twisted   product structure
  We characterize Osserman and conformally Osserman Riemannian manifolds with the local structure of a warped product. By means of this approach we analyze the twisted product structure and obtain, as a consequence, that the only Osserman manifolds which can be written as a twisted product are those of constant curvature. 
M. Brozos-Vazquez, E. Garcia-Rio, R. Vazquez-Lorenzo