Overgods are overgods. Presumably when they enter the domain of another overgod, they'd be measured against that overgod (and it's only polite to let the other guy do what he wants in his own "project"), but they'd still be overgods.
Ao does have a boss, so there are greater overgods than him. Realmspace is just his personal project, and he says himself that there are countless others that (presumably) other overgods are overseeing.
"The Wild Mages I have met exhibit a startling disregard for common sense, and are often meddling with powers far beyond their own control." ~Volo "Not unlike a certain travelogue author with whom I am unfortunately acquainted." ~Elminster
He most likely be significantly more powerful than the local deities but Eldacar pointed out we cannot know for sure if he would be more powerful than that plane's local overgod.