Add repository and import GPG key
sudo dnf -y install https://yum.opennms.org/repofiles/opennms-repo-stable-rhel8.noarch.rpm
sudo rpm --import https://yum.opennms.org/OPENNMS-GPG-KEY
Install Horizon with all built-in dependencies
sudo dnf -y install opennms

If you want time series trending and forecast functions you must install the R project packages. The additional download size for packages is ~390 MB.

Install R-core packages for time series trending and forecasting (optional)
sudo dnf -y install epel-release
sudo dnf -y install R-core
Disable the OpenNMS Horizon repository after installation to prevent unwanted upgrades when upgrading other packages on the server. After upgrade, Horizon requires manual steps to upgrade configuration files or migrate database schemas to a new version. We recommend that you exclude the Horizon packages from update except when you plan to perform an upgrade.
Disable auto updates for OpenNMS Horizon
sudo dnf -y install yum-utils
sudo dnf config-manager --disable opennms-repo-stable-*
Verify directory structure with the tree command
sudo dnf -y install tree
tree /opt/opennms -L 1
Directory structure after successful installation
/opt/opennms
├── bin
├── contrib
├── data
├── deploy
├── etc
├── jetty-webapps
├── lib
├── logs -> /var/log/opennms
├── share -> /var/opennms
└── system