/etc/resolv.conf said it was generated by Connection Manager.
I finally found this thread and figured out how to set the nameservers for each connection by using connmanctl. Then, I restarted the connman.service. Now, the OpenNIC IP addresses are in /etc/resolv.conf, and my ISP does not hijack my DNS :-)
Also, identi.ca is back. I was just about to use microca.st.