I've been reading along here and it seems I have similar problems like
@Berul . My recent OCC's have been surprisingly succesful and now I know that's probably because I can play my best cards here: Advanced diplo, research order, GP generation, use of fail gold...
On the other hand, when a player is unsure of what to do with other cities, then he has nothing to gain from OCC. For example, I am really weak at settling promising spots. Sometimes, my 2. or 3. city do not contribute enough, and then in other situations, I was surprised how good a city became later that I had estimated very weak in the first place. I don't have the eye for a city's potential, when the tiles are still unimproved. I really admire the pros on YT who have a look at a piece of jungle and claim: That will be a killer spot later on!
Another important point is what to build in all the non-capitol cities, where to whip, where to generate workers/settlers. I rather do that by intuition and it was enough for several Immortal wins. But I am not really skilled at that. I love to swap tiles between cities though, that's why my games take up to 20 hours.
I guess OCC has mainly one learning effect: It can tell you the maximum about advanced GP management. All the yield differences between settled and hired guys, what wonder generates what kind of points, which GPs to get and which to avoid, how to use civics and how to get quicker to the relevant techs. That's why everyone recommands Philo.
Yeah, we lack a IND/PHI leader in that game, same for CRE/CHA. The second one should definitely be Obama