Please refer to the official Oxygen documentation: https://oxygenbuilder.com/documentation/design-library/user-design-library/

Note that the majority of our products are sold as shortcode downloads or Oxide Connector products, which are not standard design sets and require a license key.

The following is taken directly from that page:

To use a third party design set, you'll need to have the site key for that design set. To install it on your site, go to Oxygen > Settings > Library in the WordPress admin panel and tick the Enable 3rd Party Design Sets box, then click Update.

Next, click + Add Design Set and enter the Site Key of the design set you'd like to add.

Click Add Source Site. Now the new design set will be available under +Add > Library in the Oxygen builder. You can also install the entire design set from the site installation wizard, accessible via Oxygen > Home in the WordPress admin panel.