FWIW, the USNA class of 2017 had an acceptance rate of just under 7% (a class size of 1,206 from 17,654 applicants
source). The Pilot/NFO board that met in February accepted 48% of its applicants (if the gouge that somebody posted in that thread is accurate). I haven't been able to find the acceptance rate from the April board, but I would imagine it was similar.
I think there is a reporting bias in the stats posted on this site towards reporting high scores, which would make OCS seem more competitive than USNA/ROTC. Also, applicants here probably score better on the ASTB than the applicant field as a whole because of all the resources available on AW.