On page: v3.1 Release Notes — Open OnDemand 3.1.0 documentation
RedHat/Rocky Linux/AlmaLinux 8 command actually shows the URL for EL9, not 8:
sudo yum install -y https://yum.osc.edu/ondemand/3.1/ondemand-release-web-3.1-1.el9.noarch.rpm
Thanks! That should be fixed in just a second. Note that there were 2 EL8 tabs when one of them should have been el9.