UIApplication.sharedApplication() is not accessible from iOS application extension

Do deactivate codes that use UIApplication.sharedApplication() your must add -D OAUTH_APP_EXTENSIONS to your OTHER_SWIFT_FLAGS on OAuthSwift target

You must also set a custom authorize_url_handler to your oauthswift object, the default one will not work (You can try the ExtensionContextURLHandler)

For Cocoapods

Add at the end of your podfile something like that

post_install do |installer|
    installer.pods_project.targets.each do |target|
        if target.name == "OAuthSwift"
            puts "Updating #{target.name} OTHER_SWIFT_FLAGS"
            target.build_configurations.each do |config|
              config.build_settings['OTHER_SWIFT_FLAGS'] ||= ['$(inherited)']
              if !config.build_settings['OTHER_SWIFT_FLAGS'].include? " \"-D\" \"OAUTH_APP_EXTENSIONS\""
                config.build_settings['OTHER_SWIFT_FLAGS'] << " \"-D\" \"OAUTH_APP_EXTENSIONS\""
              end
            end
        end
    end
end