In the development of O-RAN based systems, interoperability should receive more attention. The O-RAN specification is becoming more and more complex and permissive, increasing the odds of incompatibility issues between O-RAN devices that are supposed to work together. In practice, interoperability is not something that can only be verified late in the system integration phase, but devices should be designed and built with interoperability as the key driver. So far, interoperability tests have only been possible in a very late phase, when both the O-DU and O-RU have met each other in an integration lab. This has made interoperability testing a tedious trial-and-error effort, imposing risks and delays for O-RAN based multi-vendor integrations.