My biggest project that used OOP principles - A shop where a client must be created/or chosen if needed and then, the client must buy some products. The products are tagged via EAN8 or EAN13. When the client buy the products, a new form with the invoice will appear with the products and the total prices (with a 19% VAT).
I implemented EAN 8, EAN 13 and QR Codes in order to tag the products that the user was going to buy. I also created an Invoice page, where the user could save and print the invoice.