I doubt DNS has anything to do with security. If you want to, think of it as a phonebooks consisting of addresses of various servers.
The difference between ISP's default DNS server and OpenDNS would be same as using a old phonebook printed five years ago against using a current one. Both may still have good addresses for popular & bigger players such as Google, but the older phonebook may have invalid entries that requires even more lookups in other DNS server before you reach the destination.
So there's no change to security here. As for privacy, I would think it's a case of switching out from ISP's DNS server to OpenDNS, so your browsing activity is now on a different server, if they do even keep that kind of information. (and I don't know for a fact that they don't or do)
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