GromBeestje

The OAuth plugin for WordPress allows you to provide authentication against any 
OAuth provider. This plugin is uses the OAuth library by Manuel Lemos. The 
plugin allows the admin to select any from the services supported 
out-of-the-box by the said library. Additionally it is possible to configure a 
service manually, which means any site supporting the OAuth protocol can 
be used.

BlaatLogin: OAuth

This plugin turns your WordPress website into an OAuth Consumer. It allowsallows your users to sign in with any OAuth provider.

rtCamp

Shalin Shah

Sagar Tamang

Nikhil Joshua

Chirag Mathur

Ultra minimal plugin to let your users login to WordPress applications using their Google accounts. No more remembering hefty passwords!
<h3>Initial Setup</h3>
<ol>
<li>
Create a project from <a href="https://console.developers.google.com/apis/dashboard" rel="nofollow ugc">Google Developers Console</a> if none exists.
</li>
<li>
Go to Credentials tab, then create credential for OAuth client.
<ul>
<li>Application type will be Web Application</li>
<li>Add <code>YOUR_DOMAIN/wp-login.php</code> in Authorized redirect URIs</li>
</ul>
</li>
<li>
This will give you Client ID and Secret key.
</li>
<li>
Input these values either in <code>WP Admin > Settings > WP Google Login</code>, or in <code>wp-config.php</code> using the following code snippet:
<code>define( 'WP_GOOGLE_LOGIN_CLIENT_ID', 'YOUR_GOOGLE_CLIENT_ID' ); 
define( 'WP_GOOGLE_LOGIN_SECRET', 'YOUR_SECRET_KEY' );</code>
</li>
</ol>
<h3>Browser support</h3>
<a href="https://developers.google.com/identity/gsi/web/guides/supported-browsers" rel="nofollow ugc">These browsers are supported</a>. Note, for example, that One Tap Login is not supported in Safari.
<h3>How to enable automatic user registration</h3>
You can enable user registration either by 
– Enabling Settings > WP Google Login > Enable Google Login Registration
OR
<ul>
<li>Adding 
<code>define( 'WP_GOOGLE_LOGIN_USER_REGISTRATION', 'true' );</code> 
in wp-config.php file.</li>
</ul>
Note: If the checkbox is ON then, it will register valid Google users even when WordPress default setting, under
Settings > General Settings > Membership > Anyone can register checkbox
is OFF.
<h3>Restrict user registration to one or more domain(s)</h3>
By default, when you enable user registration via constant <code>WP_GOOGLE_LOGIN_USER_REGISTRATION</code> or enable Settings > WP Google Login > Enable Google Login Registration, it will create a user for any Google login (including gmail.com users). If you are planning to use this plugin on a private, internal site, then you may like to restrict user registration to users under a single Google Suite organization. This configuration variable does that.
Add your domain name, without any schema prefix and <code>www,</code> as the value of <code>WP_GOOGLE_LOGIN_WHITELIST_DOMAINS</code> constant or in the settings <code>Settings > WP Google Login > Whitelisted Domains</code>. You can whitelist multiple domains. Please separate domains with commas. See the below example to know how to do it via constants: 
 <code>define( 'WP_GOOGLE_LOGIN_WHITELIST_DOMAINS', 'example.com,sample.com' );</code>
Note: If a user already exists, they will be allowed to login with Google regardless of whether their domain is whitelisted or not. Whitelisting will only prevent users from registering with email addresses from non-whitelisted domains.
<h3>Hooks</h3>
For a list of all hooks please refer to <a href="https://github.com/rtCamp/login-with-google#hooks" rel="nofollow ugc">this documentation</a>.
<h4>wp-config.php parameters list</h4>
<ul>
<li>
<code>WP_GOOGLE_LOGIN_CLIENT_ID</code> (string): Google client ID of your application.
</li>
<li>
<code>WP_GOOGLE_LOGIN_SECRET</code> (string): Secret key of your application
</li>
<li>
<code>WP_GOOGLE_LOGIN_USER_REGISTRATION</code> (boolean) (optional): Set <code>true</code> If you want to enable new user registration. By default, user registration defers to <code>Settings > General Settings > Membership</code> if constant is not set.
</li>
<li>
<code>WP_GOOGLE_LOGIN_WHITELIST_DOMAINS</code> (string) (optional): Domain names, if you want to restrict login with your custom domain. By default, it will allow all domains. You can whitelist multiple domains.
</li>
</ul>
<h3>BTW, We’re Hiring!</h3>
<a href="https://rtcamp.com/careers/" rel="nofollow ugc"></a>

Log in with Google

In terms of ready-to-go possibilities, this plugin is the best free plugin for both Logging in and Registering with Google.

BlaatLogin: OAuth vs Log in with Google