OPA takes a whole different approach. They basically have a specific language and a corresponding interpreter. The previous version of Keto was based off of OPA. We discovered that it is not suitable for our needs and what we want to offer because it can be very hard to meet certain performance goals (e.g. https://github.com/ory/keto/issues/104)

Taking this simple approach allows us to provide a global, low latency, and consistent authorization system.