Joseph-
“If we agree O.P. is a reshus harabim mdoraisa”
– Sorry, I don’t remember. I seem to recall it’s a safek, but I just don’t know.
“what constitutes a ‘break’?”
– To the best of my recollection, an overpass or something else that prevents other streets from being mitztaref with Ocean Parkway.
“Why is Queens (or even Nassau) not affected by O.P.? Brooklyn and Queens are both part of the same city.”
– If I remember correctly it is because there are overpasses orother breaks on all roadways that are wide enough that would allow tziruf.
I am not comfortable discussing this, because my knowledge of the issues is so incomplete, and it is halacha lemaseh.
There are many rabonim on both sides of the eiruv issue who are knowledgeable re: what’s involved and probably should be asked these questions.