I'm assuming you're referring to the type inferencing? That is done at compile time, not at runtime. Opa is statically typed.
I'd like to add that for me, the biggest plus for the language is that it lets you avoid the details of hassling with ajax requests and trying to push notifications to clients by completely abstracting all that away. Compared to most other web development languages/frameworks you know that you can write your entire web app in one language and that if within 1 function you want to persist something to the database and update the webpage DOM you can do that and the opa compiler will handle splitting that out into the client-side and server-side code to accomplish that with the glue to hold them together. That is what makes the language beautiful to me.