In this article, I want to share my personal experience on the successful migration of Oxygen sites. Also, these steps can be applied when upgrading Oxygen.
I will not go into detail about migration plugins (this is a topic for another conversation) and here we will only talk about the sequence of actions.
Before updating Oxygen or before the site migration process, be sure to clear the browser cache and server cache, disable the caching plugin and the caching system on the server.
If, after completing all the steps, you still see a broken layout on the frontend, then try repeating steps 3 through 5. In rare cases, repeating these steps worked for me.
An important point: make sure the problem is not related to PHP memory limit on your server (check the settings on the server and increase the limit if necessary). Also I do not recommend using Oxygen on PHP 8.x.x. Still stable on PHP 7.3 and I stay with that.
If after migrating the site you still have problems with incorrect URLs for images and files, then use the Better Search Replace plugin for bulk replacement.
If the above algorithm does not help, contact Oxygen support. When contacting support, explain the problem in detail and immediately provide access to the sites. You will usually get help within 10-48 hours.