How do I open an URL from my OpenGL application without leaving the application?
Is there an easy way to in-app-open an URL just like AdMob does as we click the banner?
Have a look at UIWebView.
You need to create a new controller with uiwebview, and in the button action target for the new controller.
By OpenGL application do you mean you haven't got a handy navigation controller around to push a uiwebview onto it?
If so, you can create a navigation controller, hide it in your app (move it off screen). Push your uiwebview onto it (remembering to bring the navigation controller back into view. You need to do this so the user can return to your app).