Yes, you can drop in .ml files and just use OCaml syntax.
Constructs that are easy to express in ReScript are oftentimes inconvenient in OCaml syntax. E.g. there’s no JSX, so you need to use plain functions instead to handle your react element (basically write in the desugared way).
Biggest downside is that the docs are not maintained in OCaml syntax, so you’ll have a hard time finding resources online.