Can you write effective unit tests without knowlegde of designing ?
I think that Jeremy is right in saying no.
OOP is all about design.
Concepts like Object, Collaboration and Messaging or Inheritance,Polymorphism and Encapsulation are all about design.
If a person does not understand OOP,Domain Driven Design, he can hardly write effective unit tests.
In OOP , one has to take 100 design decisions(which code goes to which class or layer) per day.
TDD is advance concept to master. One has to learn OOP and Design Patterns before that.