At least superficially, this seems close to UR/Web. (http://www.impredicative.com/ur/). I assume you guys are aware of each other. Can you tell us something about the advantages of Opa?
Ur/Web syntax seems closer to OCaml syntax, so it might be a bit easier to learn. However, Opa documentation seems more polished and complete so it might even the odds.
Nice work, and thanks for bringing static guarantees to web development. It is much needed.