This document explains how to customize the AJAX loader displayed in the Owl Carousel Pro plugin in the popup window when the click action is changed to Open Post in Popup.
The example below replaces the default loader with a simple rotating circle:
You can replace the HTML inside the dp-ocp-loader div and accompanying CSS with any loader you want. You’ll find a few other examples of preloaders here. Keep in mind these examples are for the Divi FilterGrid plugin. If you use these examples, you’ll want to change the filter, function and div class to match the code above on the current page.