Google's DNS has been working all day here. The problem is that Dyn's DNS server is being DDoS'd; if you request a record that the authoritative DNS server for is hosted by Dyn, then when you query Google's DNS for that record, then Google's server needs to make a query to Dyn, which is down, and thus, your query fails. But queries to Google for non-Dyn domains will continue to work just fine.

OpenDNS works because, as another poster notes, that, for better or worse, they don't strictly obey TTLs: https://news.ycombinator.com/item?id=12762429