Indeed the package for OpenSuse (and all other packages I think) is unsigned so you would need to ignore the warning when it arrives. The steps would be (assuming the installer is in Downloads):
noob:~/Downloads> sudo zypper install ooRexx-5.0.0-12583.opensuse15.x86_64.rpm
[sudo] password for root:
Loading repository data...
Warning: Repository 'Update repository of openSUSE Backports' appears to be outdated. Consider using a different mirror or server.
Warning: Repository 'Update Repository (Non-Oss)' appears to be outdated. Consider using a different mirror or server.
Reading installed packages...
Resolving package dependencies...
The following NEW package is going to be installed:
ooRexx
1 new package to install.
Overall download size: 1.0 MiB. Already cached: 0 B. After the operation,
additional 6.6 MiB will be used.
Continue? [y/n/v/...? shows all options] (y): y
Retrieving package ooRexx-5.0.0-12583.x86_64
(1/1), 1.0 MiB ( 6.6 MiB unpacked)
ooRexx-5.0.0-12583.opensuse15.x86_64.rpm:
Package header is not signed!
ooRexx-5.0.0-12583.x86_64 (Plain RPM files cache): Signature verification failed [6-File is unsigned]
Abort, retry, ignore? [a/r/i] (a): i
After that it should work.
(Note: there is a minor bug here, the name of the installed package should be oorexx to follow convention this is fixed in the running release 5.1.0Beta)
Good luck.