Significance of Object-Oriented Programming (OOP) in Data Science
It actually depends on the role you get as a Data Scientist. If you have to write production-quality code at a large software company, then you need to be knowing the basics of Object-Oriented Programming (OOP). Object-Oriented Design (OOD), however, is something you need not necessarily know in a Data Science role. Learning OOD in case you plan a switch to software engineering roles in future is one thing you can consider, though.
Regarding the choice of language for doing Data Science, I would suggest you  prefer Python over R as it's more versatile and a much more general purpose language. While R, on the other hand, is limited only to applications in statistical programming.