Currently when you are using OAuth according to the instructions here, the Step 3 is to "Redirect the user to Bitbucket to authorize your application". Now, the page that the user is redirected to is, by default, the login page. On a mobile browser it's a quite terrible user experience: the user must zoom in and scroll around just to log in!
Please make the login page to scale gracefully to small screens. If the current page cannot be made to do that, please create a new page and allow the authentication URL requester to specify the type of login page they want as a parameter in the call.
(Attached: screenshot from Chrome for Android on Samsung Galaxy Nexus.)