I had a similar problem (on MacOS 10.10.2, openssl 1.0.2 and offlineimap 6.5.5 both from homebrew) and couldn’t get the dummy certificate solution to work. However, I found a certfile that makes offlineimap stop complaining in /usr/local/etc/openssl/cert.pem (which is put there during the installation of openssl through homebrew, brew info openssl tells me). Caveat: I don’t know whether this is a good or safe way to do this.