Optimal settings? Well, like vmxa implied, you can make a OCC pretty dang easy by using a tiny map, high water %, etc. The question is, is what is optimal to you? What sort of map size and difficulty level did you want to try a OCC on and still have it be doable?
Maybe to give you an idea...
Recently I've been
trying 
OCC AW Emperor Standard Pangea games. This is a
insanely difficult variant, and you have to have a super-high powered start that includes most or all of the following:
- Coastal
- Fresh water, river preferred for extra commerce
- 2-3 food bonuses, preferably cattle for the shield bonus
- At least 2-3 hills/mountains in the fat X.
- Nearby luxes or resources, especially ivory, is very helpful.
The goal is to be able to do at least 5fpt / 20spt in despotism, and a dream start allows 30spt in a non-despotic government
Pick a civ with an excellent UU and good traits/starting techs. Starting techs are a huge consideration especially if you allow initial trading before you declare war. Regardless, having Alphabet to begin with gives you the highest cost starting tech and gives you a jump on the road to Literature/GLib.
I don't care too much who the AI civs are, although having Greece or Carthage can be royal pain because they will send waves of 3 def units at your lands to pillage. I do this :suicide: every time I see a stack of Hoplites coming for my land. I started picking my opponents after I drew Greece as a a neighbor like 3 games in a row.
IMO, good traits for OCC are Industrious, Militaristic, Seafaring and Science. Religious and Agricultural are ok. Expansionistic and Commercial are almost useless, except for Commercial starting with Alphabet.
Wonders are also super important to build, both for their effects and for their tourism commerce. Key wonders include Colossus (hence the need for a coastal start), SoZ if you have ivory, and the Great Library. The MoM can also be nice.
If you play an easier version than the one I described, you can do with a poorer start and an average quality civ.
