Dear Sabine,
Firstly, you should have at least one person who know object oriented programming logic and patterns in your team. If you do not have a senior OOP developer in project, you cannot understand that is it the right way or not. One senior developer can be a control point and evaluate project process technically.
From technical perspective there are lots of documents for OO-Design Patterns in the internet. It is not necessary to learn with ABAP to understand design patterns. You can also use Java, C# or C++ documents for design patterns. However there is a SAP PRESS book for ABAP-OO Design patterns. Here is the link; Design Patterns in Object-Oriented ABAP. von Igor Barbaric - by SAP PRESS
And also there is another book for Object Oriented ABAP powered by SAP PRESS again; Object-Oriented Programming with ABAP Objects. von Ja - by SAP PRESS
This book will be enough to learn and apply your learnings to business process. Your team just need some time to handle OO design patterns. If you have experienced team in development, it will not take so much time to understand.