diff --git a/lib/phpmailer/readme_moodle.txt b/lib/phpmailer/readme_moodle.txt
index f57db50c66d..e7876755ab5 100644
--- a/lib/phpmailer/readme_moodle.txt
+++ b/lib/phpmailer/readme_moodle.txt
@@ -13,6 +13,5 @@ To upgrade this library:
    - composer.json
    - get_oauth_token.php
    - SECURITY.md
-   - src/OAuth.php
    - src/POP3.php
 5. Update lib/thirdpartylibs.xml.
diff --git a/lib/phpmailer/src/OAuth.php b/lib/phpmailer/src/OAuth.php
new file mode 100644
index 00000000000..c93d0be1b55
--- /dev/null
+++ b/lib/phpmailer/src/OAuth.php
@@ -0,0 +1,139 @@
+<?php
+
+/**
+ * PHPMailer - PHP email creation and transport class.
+ * PHP Version 5.5.
+ *
+ * @see       https://github.com/PHPMailer/PHPMailer/ The PHPMailer GitHub project
+ *
+ * @author    Marcus Bointon (Synchro/coolbru) <phpmailer@synchromedia.co.uk>
+ * @author    Jim Jagielski (jimjag) <jimjag@gmail.com>
+ * @author    Andy Prevost (codeworxtech) <codeworxtech@users.sourceforge.net>
+ * @author    Brent R. Matzelle (original founder)
+ * @copyright 2012 - 2020 Marcus Bointon
+ * @copyright 2010 - 2012 Jim Jagielski
+ * @copyright 2004 - 2009 Andy Prevost
+ * @license   http://www.gnu.org/copyleft/lesser.html GNU Lesser General Public License
+ * @note      This program is distributed in the hope that it will be useful - WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.
+ */
+
+namespace PHPMailer\PHPMailer;
+
+use League\OAuth2\Client\Grant\RefreshToken;
+use League\OAuth2\Client\Provider\AbstractProvider;
+use League\OAuth2\Client\Token\AccessToken;
+
+/**
+ * OAuth - OAuth2 authentication wrapper class.
+ * Uses the oauth2-client package from the League of Extraordinary Packages.
+ *
+ * @see     http://oauth2-client.thephpleague.com
+ *
+ * @author  Marcus Bointon (Synchro/coolbru) <phpmailer@synchromedia.co.uk>
+ */
+class OAuth
+{
+    /**
+     * An instance of the League OAuth Client Provider.
+     *
+     * @var AbstractProvider
+     */
+    protected $provider;
+
+    /**
+     * The current OAuth access token.
+     *
+     * @var AccessToken
+     */
+    protected $oauthToken;
+
+    /**
+     * The user's email address, usually used as the login ID
+     * and also the from address when sending email.
+     *
+     * @var string
+     */
+    protected $oauthUserEmail = '';
+
+    /**
+     * The client secret, generated in the app definition of the service you're connecting to.
+     *
+     * @var string
+     */
+    protected $oauthClientSecret = '';
+
+    /**
+     * The client ID, generated in the app definition of the service you're connecting to.
+     *
+     * @var string
+     */
+    protected $oauthClientId = '';
+
+    /**
+     * The refresh token, used to obtain new AccessTokens.
+     *
+     * @var string
+     */
+    protected $oauthRefreshToken = '';
+
+    /**
+     * OAuth constructor.
+     *
+     * @param array $options Associative array containing
+     *                       `provider`, `userName`, `clientSecret`, `clientId` and `refreshToken` elements
+     */
+    public function __construct($options)
+    {
+        $this->provider = $options['provider'];
+        $this->oauthUserEmail = $options['userName'];
+        $this->oauthClientSecret = $options['clientSecret'];
+        $this->oauthClientId = $options['clientId'];
+        $this->oauthRefreshToken = $options['refreshToken'];
+    }
+
+    /**
+     * Get a new RefreshToken.
+     *
+     * @return RefreshToken
+     */
+    protected function getGrant()
+    {
+        return new RefreshToken();
+    }
+
+    /**
+     * Get a new AccessToken.
+     *
+     * @return AccessToken
+     */
+    protected function getToken()
+    {
+        return $this->provider->getAccessToken(
+            $this->getGrant(),
+            ['refresh_token' => $this->oauthRefreshToken]
+        );
+    }
+
+    /**
+     * Generate a base64-encoded OAuth token.
+     *
+     * @return string
+     */
+    public function getOauth64()
+    {
+        //Get a new token if it's not available or has expired
+        if (null === $this->oauthToken || $this->oauthToken->hasExpired()) {
+            $this->oauthToken = $this->getToken();
+        }
+
+        return base64_encode(
+            'user=' .
+            $this->oauthUserEmail .
+            "\001auth=Bearer " .
+            $this->oauthToken .
+            "\001\001"
+        );
+    }
+}