This diagram could be a bit difficult to understand at first and follow all the relations. To be able to use an interactive version of the same diagram, please use the MySQL Workbench and follow these steps.
Download MySQL Workbench (it's free and cross-platform!)
Install it and open it
Select Create EER Model From SQL Script
Look for your file 'struct.sql (your_osclass_folder/oc-includes/osclass/installer/struct.sql)
Check Place imported object on a diagram
Click on Execute, Next, Finish
Re-arrange the boxes (tables) as you wish
Relations will highlight if you place your mouse over the tables or the relations.