OCSP works until it is needed. See in https://www.imperialviolet.org/2012/02/05/crlsets.html the comparison to the seat-belt. A hacker which can do a MITM attack (the main reason to have certificates), can block access to the OCSP server in all important cases. Thus without a warning that the OCSP server isn't reachable, OCSP is completely useless because it fails exactly when it is needed (someone is attacking you).
