Well the most logical is that vader would appear as hologram to ozel.
Taking the fact that where ozzel and piet is located, behind them is the location that later in the movie the 3 admirals report to vader as holograms and Needa dies.Thats why Ozzel comes closer to that point.
So again assuming the system that is from screen to holo and holo to holo.
Maybe Impfigher has something to say :D
-Angel