let greet subject =
  match subject with
  | None    -> "Hello, World!"
  | Some(s) -> String.concat "" ["Hello, "; s; "!"]

let greet subject =
  match subject with
  | None    -> "Hello, World!"
  | Some(s) -> "Hello, " ^ s ^ "!"

Use ^ as concatenation operator
Use ^ as concatenation operator

As suggested by stevejb71
