Oxygen does not support WordPress wp_body_open() and if you need to integrate Google Tag manager it requires two code parts.
1st part is Inside Head and 2nd is inside Body.
Lucky Oxygen provides hook that is called: ct_before_builder.
If you are using Scripts Organizer , Code Snippets or any other functionality plugins this script will be beneficial for you.
<?php function custom_content_after_body_open_tag() { ?>
<!-- Your custom code goes here. -->
<div>My Custom Content</div>
<!-- Your custom code goes here. -->
<?php
} add_action('ct_before_builder', 'custom_content_after_body_open_tag');
?>