I used Chern for OKAPs and Friedman for the real Boards. Friedman is by far the more thorough book, so if you have to pick one, I'd go for that one.
I didn't study much for the OKAPs because I did not want to do a fellowship, thus focused more of my time and energy on clinical stuff rather than spending time on an exam that doesn't count for anything. I used Chern because it was shorter and easier to read. I don't think I even owned Friedman until after I finished residency.
For the actual boards, the only resources I used were the Friedman book (which I basically memorized), the Kanski (just as an atlas), and the practice questions found in the back of the BCSC. I ended doing really well on the written boards despite having mediocre to low OKAP scores as a resident.
