To enable client scripts invoke REST services, different API providers have option called CORS profile which enable/allows HTTP call from cross domain from browser script.
Below page shows how to enable CORS profile in Oracle API Gateway, enabling REST call from a client(AJAX) fro a different domain.
Page Link : CORS Profile in Oracle API Gateway (OAG)
Like it if you have learned something. Comment your views, queries. Any suggestion and corrections are always appreciated. Keep learning.