I sort of have to agree with @Ivan Seeking insofar as the ambiguous nature of what was actually discovered in the OA.I think it would be a pretty different show if half of it wasn't about a scientist with advanced tech that is able to monitor near death experiences trying to prove the afterlife.
Yes, it's true whatever was discovered was done so by a scientist with advanced tech, monitoring near death experiences, trying to prove the afterlife. But as the series progresses, the scientist's hypothesis is proven wrong. Whatever was discovered was not the afterlife -- at least not an afterlife of a traditionally believed nature. It was something else. Something different. What exactly it is isn't clear.
So yeah, there's science in the show. But the "science" part is debunked, at least a little, as part of the plot. So is that still science fiction? Maybe, but not necessarily certainly.
It would seem that what was actually discovered was some sort of proof of an Everttian interpretation of quantum mechanics (QM). But was it? The show could have done some hand-waving along those lines bringing it solidly back into the realm of science fiction. But they didn't actually do any invocation of QM. Well, not yet anyway.
So I guess we'll see, if there's another season.