How do you omit byte []?

Question

    let f (o:obj option) = 
        o
        |> Option.bind(
            function 
                | :? byte[] as b -> Some b 
                | _ -> None)

but it does not compile

the following may fail at runtime, and not only to be used if it is the correct type, so that it does not match the desired behavior

let f (o:obj option) = o |> Option.map(fun b -> b :?> byte[])

How can you try a objto byte[]?