Originally written some time in 2016
While setting up a site for a client, I ran into some trouble with OpenCart and HTTPs. Apparently, for whatever reason, the developers decided to only use HTTPS for the checkout portion of the site. For an e-commerce site, this was completely unacceptable for my client, as we wanted EVERYTHING on that site to use HTTPs. So finally after some time spent with Google, I discovered a wonderful forum post. Here is the link.
The poster said to do the following, and sure enough, it worked!
Look in upload/system/config/catalog.php and upload/system/config/admin.php. Set $_['site_ssl'] = false; to $_['site_ssl'] = true;
Now all pages successfully load over HTTPs.