Dear Justin,
Welcome to the forum here.
Please note that SPO only supports WCF and SQL Server Azure Connectors and you may use SharePoint Designer 2013 to create the BDC Model.
Introduction to external data - Microsoft Support
I'm afraid that there may be no easy way connecting PostgreSQL to SharePoint Online.
As what you are investigating is a SharePoint development question and we have a specific channel sharepoint-dev - Microsoft Q&A coping with SharePoint Development issues and questions, I kindly suggest you post a new thread to see if there is a way achieving your requirement.
Thanks for your understanding and have a nice day!
Sincerely,
Cliff | Microsoft Community Moderator