First off, I'm 90% certain that ou-0 just means the bottom of the ladder at 1000, shortened due to technical restrains or something. Other than that, yes, you are correct in assuming they are records for different ladder ratings.
They exist because some parts of the ladder are different from other parts in terms of their skill level and what they use. For example, in the lower ladder you might find a lot of people who are still learning the tier and the calcs and all that, which is why I'm guessing Sawsbuck and Noctowl are on that OU-0 list (they aren't even close to the bottom either). You might even run into a lot of youtubers who use troll teams on the bottom of the ladder.
Closer to the top of the ladder, games become a lot more serious and people try harder. That is why you'll usually find better results for what is good in the tier near the top, because all the bad Pokemon will not even be in the list due to their losses in the lower ladder.
If you've ever watched a youtuber (I know most people don't have the time) like PokeaimMD and his Road to Top 10 series, he constantly talks about the different skill levels at various points in the ladder. He often calls the bottom of the ladder the scariest part because you never know what they will use or what they will do.
I hope this helps, I'm not very good at explaining competitive sometimes so ask if you have anymore questions.
