I mean, I have an OOP background. I found FP as a result of my dissatisfaction with OOP. In fact, I used to teach OOP languages to new students and saw the same mistakes over and over again, mistakes that are simply not possible in FP. It's a very similar story for everyone I work with, too. We all had jobs in various OOP languages before we managed to get jobs writing Haskell.

Oh, and I'm currently teaching Haskell to someone at work who has a CS degree and has only done OOP languages (and C), and while it's different than what he's used to, he's still picking it up very quickly (working towards making him a junior engineer, which I think shouldn't take too much longer). In fact, just the other day we pair programmed on a bug ticket I have and he was not only following along with the code, he spotted issues I hadn't seen yet. Part of it is certainly that's he smart (which is why I'm doing this in the first place), but part of it is also that, with a bit of familiarity, FP languages are incredibly easy to read and follow. The primary difference is that FP does everything explicitly, whereas OOP encourages a lot of implicit (and hidden) behavior. When you organize code around functions, there's necessarily more explicit arguments and explicit return values, which makes it far, far easier to follow the flow of logic of code (and test!). Recently I was trying to read through our Kotlin codebase at work (for our Android app), and it was so much harder because so much is implicit.