The majority of OpenNIC documentation is located here, on the wiki.
How to use OpenNIC DNS: http://wiki.opennicproject.org/GettingStarted/
For further information, you might also be interested in these links:
https://servers.opennicproject.org/ — View the entire list of public servers and see more detailed information about each
http://report.opennicproject.org/ — Check here for server outages, historical information, and global maps
https://api.opennicproject.org/geoip/?help — There are a number of options available for obtaining lists of recommended servers near you
Note that a number of public servers are marked as “whitelisting”. These servers are not open for general public use, but rather require you to register your IP address with OpenNIC. These servers will have lower traffic and fewer problems with DDoS attacks, allowing for greater reliability. You can find more information about this service on our wiki page.