Yes. OpenNMS is 100% open source and all tooling is available via open source resources. We provide all the build tooling for developing against the Java APIs. There's also a nice API for developing plugins that doesn't require a full rebuild of OpenNMS—or even starting or stopping the system to install the plugin. 

We use CircleCI to host our CI/CD environment. Our entire platform is based on test-driven development, with as much (or more) software written in tests as we have in actual testing software. All of this is verifiable, since the tests, code, and entire CI/CD pipeline are all open source. The build pipeline from [...]

In addition to the services that GitHub and SonarCloud provide, the OpenNMS community also reports on vulnerabilities. With thousands of users and companies running the software, many in DoD, we have a lot of eyes and vulnerability scans on the software.