Yeah OCC is good practice at making super cities, which is useful even when not playing OCC. When you play an OCC and realize how well you're keeping up with the AI's even though you only have one city it makes you realize how much better you could do if you had multiple cities. Sure you can't build all the national epics in one city but you can make 2 or 3 super cities instead. I definitely learned a lot with my OCC noble win. I didn't even have a good city location (had a ton of coastal tiles and only 1 hill).
OCC's are fun also because the game goes by so quickly since you only have the one city to manage.