I can't find my D&D book, but IIRC they have all of the information you need to create your gods. An Overdeity is not mentioned to have any super cosmic abilities. Again, IIRC he just has the only divine power, so there is no way for any other gods to exist or something like that. But of course they can, since everyone likes to "Homebrew", as the younger people call it now days.
Besides Overdeities are not in 5e anyway. It appears that the Greater gods are depicted as an Overdeity from the definition pg 11:
Greater gods: Are beyond mortal understanding. They cannot be summoned, and they are almost always removed from direct involvement in mortal affairs. On very rare occasions they manifest themselves in Avatars like lesser deities, but the slaying of the avatar has no direct affect on the deity himself.
Overgods are in 5e, both AO and the Lady of Pain are mentioned in books. One interesting thing about Overgods that people are forgetting is that Overgods hate being worshipped and often actively punish/and or discourage it. That is beyond power what makes them different from other Gods. Why this is I do not know.