The issue/bug was described in oakserver/oak#400
To check how FormData works fine in Deno, use deno run ./DenoServer.js and open localhost:8000 in the browser. It will show the response parsed as FormData correctly.
Instead, deno run ./OakServer.js sends the FormData, but for some reason web browsers can't parse it as FormData. Firefox logs the error: TypeError: Could not parse content as FormData and in Chrome says: TypeError: Invalid MIME type.
In any case, curl -X POST http://localhost:8000/formData will retrieve the FormData using ./DenoServer, and also ./OakServer.js.