@Hinidu I've overlooked this line. Formerly this has been `0` for success, `-1` for failure, now it's `1` for success and `0` for failure. I think this is the return value, when you call `rename` from ex-mode (see `:help rename`). So if we decide to break this behaviour here, we have to 1) adjust the return value two lines above as well, 2) correct the help-text of `rename`.

But I'd strongly recommend not to break this here, since this would probably break some plugins. So maybe just add an if-clause here which translates the return value.
`f_rename` gets a little uglier but I think it's still beneficial to use `OK` and `FAIL` instead of `0` and `-1` in `os_rename`.