@Nuwan Arambage
I think we have plenty of sites and tons of articles on designs / best practices ...
You aready are decently aware of OO designing principle when you say you know
Open closed Principle
Liskov Substitution Principle
Dependency Inversion Principle
Interface Segregation Principle
Composite Reuse Principle
Principle of Least Knowledge
I bet still many experienced developer might not be knowing all these...
Gof is a considered as a Bible for Designs... you are already aware of it
For
Java developer, SUN has officially published
J2EE patterns (Google)
Have a look @ Anti Patterns ....(Google )
If still you want know "best practices used in Object Oriented Programming to build extensible , more maintainable software applications......."
then I would say
You Better ask the Secrete of Success to alll billionaires in the world; it's a bit easier to get