First of all, my experience with Magento (and any kind of development) it's good to test these changes to your webshop before applying them to your live environment. But since that's easy to say afterwards, here are some of my ideas about this.
You can check the files that are modified in the file app/etc/applied.patches.list. The patch applies changes to several files. One of them is the file you mentioned. Does that file still exist and do other files that are changed by the patch (f.e. app/code/core/Mage/Oauth/controllers/Adminhtml/Oauth/AuthorizeController.php) still exist? And did you apply the correct version of the patch, since there are differences between Magento versions and the patches you need to apply.
Are you using Git or anything like that?
If this response was helpful to you, consider giving kudos to this post.
If this response solved your problem, click accept as solution to help others solve this
