I don't know the answer to that myself unfortunately. I know the entirety of the runtime itself is OSS and when you do local run/debug of a Function it's the same runtime that Azure uses.
It's not an apples to apples comparison though since OpenWhisk is intended to be self deployed.
I mentioned Functions here just because many folks don't know that most Azure SDKs, runtimes, tooling, etc are OSS by default.
That's the problem though. It doesn't matter much that it's open source if it is not usable in a sense that you can't easily deploy it yourself or repurpose, without committing (and paying) for the rest of Azure stack.
It's not an apples to apples comparison though since OpenWhisk is intended to be self deployed.
I mentioned Functions here just because many folks don't know that most Azure SDKs, runtimes, tooling, etc are OSS by default.