Adds a custom button to Oxygen Builder's TinyMCE editor for "rich text"

function.php

<?php
/**
 * Plugin Name: My Custom Button
 * Description: This is a plugin that adds a custom button to the Oxygen Builder's TinyMCE editor.
 * Version: 1.0.0
 * Author: Your Name
 */

function custom_button_plugin() {
    if (!current_user_can('edit_posts') && !current_user_can('edit_pages')) {
        return;
    }
    if (get_user_option('rich_editing') == 'true') {
        add_filter('mce_external_plugins', 'add_custom_tinymce_plugin');
        add_filter('mce_buttons', 'register_custom_button');
    }
}

add_action('init', 'custom_button_plugin');

function add_custom_tinymce_plugin($plugin_array) {
    $plugin_array['custom_button'] = plugin_dir_url(__FILE__) . 'my-custom-button.php?load_js=1';
    return $plugin_array;
}

function register_custom_button($buttons) {
    array_push($buttons, "|", "custom_button");
    return $buttons;
}

function load_js() {
    if(isset($_GET['load_js'])) {
        header('Content-Type: application/javascript');
        echo "(function() {
            tinymce.create('tinymce.plugins.custom_button', {
                init: function(editor, url) {
                    editor.addButton('custom_button', {
                        title: 'My Custom Button',
                        image: url + '/button-icon.png',
                        onclick: function() {
                            alert('You clicked my custom button!');
                        }
                    });
                },
                createControl: function(n, cm) {
                    return null;
                },
            });
            tinymce.PluginManager.add('custom_button', tinymce.plugins.custom_button);
        })();";
        exit();
    }
}

add_action('init', 'load_js');