Everything Outer Loop runs on your servers is open-source. The Mac app itself is not yet fully open-source, but the “platform” parts of it are.
This website pretty throughly documents how Outer Loop works, so anybody who really wants to could replicate it. It isn’t open-source yet, mainly because that will require some cleanup and documentation, and the first goal is to build a compelling experience before focusing on secondary goals like open-sourcing it.
Importantly, the outerframe platform is open-source, along with Outer Shell and all of its apps. Outer Loop consumes the outerframe codebase, unaltered.