As someone who has previous familiarity with both paradigms, I think Motoko is an interesting intersection of the two! I appreciate the support for naive mutability which can be a hassle in pure functional languages like Haskell. I think it will make data modelling and updates a little easier. But I also heavily appreciate the absence of “null”, being substituted for sum/variant types. This means the compiler can identify and prevent common errors that occur in most OOP languages at runtime.
I’m not sure of the helpfulness of my own advice to OOP devs, but perhaps running through a few Elm tutorials would be useful? Elm is a pure functional language with a focus on learnability, and will definitely get you up to speed with the important concepts of the functional paradigm.