I have been trying to get an OCC cultural win in G&K for a while now, so I have played a boatload off OCC games lately on immortal.
My best near losses are with the Aztecs (floating gardens) and with Korea(+2science per specialist), with Babylon a near third. I lost two games with Korea by less than 8 turns before T310. These games are really hard on immortal. I have won it easily with random civs on emperor every time I tried. OCC domination is fun at any difficulty because enemy capitols are instantly razed, so you don't have to worry about them recapturing them
For OCC the most important thing by far is population, you need to grow and be huge. Ideally you should be >36 pop, even more in BNW. Put every citizen on a food tile and only shift from total food focus to production focus to put up certain important buildings like labs. In general its best to buy buildings and build wonders when possible. The next most important thing is tech, you need to plant as many GS's as possible. If you are going for a science win you need to drag your heels on culture as much as possible until renaissance. It is important that you do not waste policies, you must fill all of tradition asap, then left rationalism, then full freedom and finish rationalism. So for science, France or any of the other natural culture civs are the worst possible civs. For cultural its the same basic idea but the timing is a bit harder to hit. Full tradition, full piety, full freedom then whatever (usually honor and liberty or commerce), I only do patronage on the rare map where there is enough food and culture CS to be worth it and the CS killers like Austria are absent.
The fun thing about OCC is that everyone loves you in most games. Since you never expand, they never get angry about you moving into their territory. I often end up being friends with all but 1 or 2 civs even if they are at war with each other. Also since you city is huge, it can pancake most units and its takes a lot of work for the AI to capture it.