Some useful TwinCAT resources I collected that focus on Object Oriented Programming (OOP):

- Stefan Henneken's blog: He writes about software design patterns and how to apply them in TwinCAT.

- Solid Principles Sketches: Wikipedia: "SOLID is a mnemonic acronym for five design principles intended to make object-oriented designs more understandable, flexible, and maintainable"

- Code smells: Wikipedia: "a code smell is any characteristic in the source code of a program that possibly indicates a deeper problem."

- PLC Coder's three pillars of OOP: "the concepts of: Encapsulation, Inheritance and Polymorphism with some examples in TwinCAT."

- Refactoring Guru: "makes it easy for you to discover everything you need to know about refactoring, design patterns, SOLID principles, and other smart programming topics."