Attempting to install an Operator into a single namespace foobar results in the error:
Operator Group does not support single namespace installation mode.
Attempting to install an Operator into all namespaces into the openshift-operators namespace results in the error:
The namespace 'openshift-operators' does not support install modes for this operator
Configuring Operator Groups doesn't change anything.
In any case, operators can't be installed outside of openshift-marketplace