Ooo is describe as a continent, and it appears that the continent is joined together and that Earth around this time has no other landmasses. Could Ooo be a super continent in the future, if this is true, the planet could have gone through plate displacement over the last thousand years, also leading to the destruction of the human species.
With Ooo as a supercontinent the climate would have been drier or maybe warmer then present day, as Antarctica doesn't cover the poles, enabling the warm air to circulate around the planet. This would have been also devastating effects on humans surviving after the mushroom war, the climate would have eliminated various lives. The climate wouldn't have just effected the human species. Modern day species would either evolve and displace their non-sapient ancestors or would gone extinct or rare. In 'Silent King' tigers and various other present day species are shown, could the goblin kingdom be the last refuge for the tigers or unchanged present days species. Maybe, or maybe not.
Maybe the humans aren't rare, maybe they live in underground bunkers, or are hiding inside other secret places that the inhabitants of Ooo aren't simply sapient about to know. Humans might be only rare in areas were Finn has gone to and that the 'ancient' fauna and humans are still common and are completely unaware of the existence of new species. Maybe on isolated islands humans survived or that they use to be common in areas were Finn gone to since there is lots of technologies and things that humans developed. Such as language, buildings and other technologies, maybe they saw humans as deities and the highest in the hierarchy. But if this is true, we're are the humans, they could have driven off by rebellions or armies of an advance Ooo civilization then took human's technologies and made it their own. Maybe, maybe not.