My answer to you was to resolve the issue you had when testing in a separate 'DIALOGUE'() and it closed all your windows when you closed the new one. Concurrent panels will take care of that.
Now, OOP is a completely different subject.
As a starter, if you want to get your feet wet with it, I would suggest creating classes as libraries of functions for your app and replace the existing calls and performs.
Then you will start using them to represent entities in your app (customers, vendors, invoices) with their data/properties (name, address, email) and their behaviours/methods (Add(), Edit(), Delete(), Purchase(), Sell(), etc...)