Syntax is fairly different but the concepts are broadly similar. Rust has many of the same nice things as F# does, like "if" as an expression rather than a statement, algebraic data types, mutability declared as a keyword, etc. In fact Rust is much stricter with the "declared mutability" thing, because in F# you can use an immutable reference to a mutable object to mutate the object.
The really big paradigm difference I found as an F# person when starting to learn Rust was the lack of guaranteed tail-call optimisation, meaning that Rust kind of wants you to avoid recursive functions. I also find it much more annoying to write sequences ("Iterators") in Rust than in F# (where the `seq` computation expression makes everything ludicrously easy at the cost of some oddly bad performance sometimes). F#'s anonymous interface implementations are also really handy, but Rust's situation there is at least no worse than C#.
Maybe I'm missing something. Under what use-cases would someone be choosing between OCaml/F# and Rust?