At present, there are two possibilities for visualization of results:
OOFEM provides a simple way to export the results in VTK format, allowing to use VTK based visualization tools (such as MayaVi or ParaView) for postprocessing on different platforms. Consult input manual on how to export data to vtk.
OOFEG - X-windows based post-processor. It requires X-windows libraries to be installed on the system, so it is more or less limited to unix platforms. Follow installation instructions on how to install and configure oofeg. The oofeg HOWTO describes its usage.