F# is very un-functional in many ways. Immutable variables are the default but mutable variables are made with just the "mutable" keyword. There are also lots of imperative control structures, like while, and for. Those two combined means that imperative algorithims typically have two distinct translations into F#.

First you can translate it literally, where F# looks like a slightly more verbose version of Python. Second you can exploit all the functional features of F#, and using immutable variables, recursion and pattern matching.

F# also has extensive support for OOP, and uses it where OCaml-ers might use modules or functors.

In that way, F# can be a sort-of stepping stone between the imperative, OOP C# to more functional features of F#.