Custom Ribbon Action Apps For SharePoint Online/O365 :

Open Visual Studio 2013> Select Option > App For SharePoint > OK



right click on on Newly created Solution > Add New Item

Select ribbon Option > 

Click ok

Modify the element .xml  based on your requirement 

<?xml version="1.0" encoding="utf-8"?>

<Elements xmlns="http://schemas.microsoft.com/sharepoint/">

  <CustomAction Id="23527d0d-e924-4838-94be-ee2e1a347210.Download Bulk Files"

                RegistrationType="List"

                RegistrationId="101"

                Location="CommandUI.Ribbon"

                Sequence="10001"

                Title="Invoke &apos;Download_Bulk_Files&apos; action">

    <CommandUIExtension>

      <!-- 

      Update the UI definitions below with the controls and the command actions

      that you want to enable for the custom action.

      -->

      <CommandUIDefinitions>

        <CommandUIDefinition Location="Ribbon.Library.Actions.Controls._children">

          <Button Id="Ribbon.Library.Actions.Download Bulk FilesButton"

                  Alt="Download Bulk Files"

                  Sequence="100"

                  Command="Invoke_Download Bulk FilesButtonRequest"

                  LabelText="Download Bulk Files"

                  TemplateAlias="o1"

                  Image32by32="_layouts/15/images/placeholder32x32.png"

                  Image16by16="_layouts/15/images/placeholder16x16.png" />

        </CommandUIDefinition>

      </CommandUIDefinitions>

      <CommandUIHandlers>

        <CommandUIHandler Command="Invoke_Download Bulk FilesButtonRequest"

                          CommandAction="~appWebUrl/Pages/ClientWebPart.aspx?{StandardTokens}&amp;SPListItemId={SelectedItemId}&amp;SPListId={SelectedListId}"/>

      </CommandUIHandlers>

    </CommandUIExtension >

  </CustomAction>

</Elements>

Write Client side Code  

<title></title>

    <script type="text/javascript" src="../Scripts/jquery-1.9.1.min.js"></script>

    <script type="text/javascript" src="/_layouts/15/MicrosoftAjax.js"></script>

    <script type="text/javascript" src="/_layouts/15/sp.runtime.js"></script>

    <script type="text/javascript" src="/_layouts/15/sp.js"></script>

    <script type="text/javascript">

        // Set the style of the client web part page to be consistent with the host web.

        (function () {

            'use strict';

            alert('Downloading !');

            var clientContext;

            var listCreationInfo;

            var web;

            var list;

            clientContext = SP.ClientContext.get_current();

            web = clientContext.get_web();

            listCreationInfo = new SP.ListCreationInformation();

            listCreationInfo.set_title("Shared Documents");

            listCreationInfo.set_templateType(SP.ListTemplateType.genericList);

            list = web.get_lists().add(listCreationInfo);

            clientContext.load(list);

            clientContext.executeQueryAsync

                (

                 //function () { alert("Success!") },

                // function () { alert("Request failed") }

                  Function.createDelegate(this, successHandler),

                  Function.createDelegate(this, errorHandler)

                );

            function successHandler()

            {

                alert('pass');

            }

            function errorHandler()

            {

                alert('fail');

            }

        })();

        

    </script>

Result