The Installation Guide references the DataStax Community Edition repo: https://docs.opennms.org/opennms/releases/latest/guide-install/guide-install.html#_newts

Per https://academy.datastax.com/planet-cassandra/cassandra, that is no longer maintained, and the instructions on http://cassandra.apache.org/download/ should be preferred.

Using the 311x repositories on Red Hat seems to work pretty painlessly; I haven't tested on Debian.