Doubt your SOG is more than about 4" thick in the field (other than grade beams), but even at that if it's properly reinforced it should be fine.
If it's got a really slick finish, what we refer to as a "hard trowel" finish hereabouts, you'd really benefit from a little scarification to rough up the surface before tiling, with or without any sort of crack-isolation or uncoupling membrane.
It is quite common to tile directly to concrete slabs. Usually it's not a problem. Sometimes the tiles crack. As Jason points out, a membrane can be added insurance against such cracking if all else is done correctly. Still no guarantees.
Porcelain tiles are ceramic tiles, by the way. Just a more dense version and what I'd prefer for my floor.
My opinion; worth price charged.