Not a product you use. The layer everything else you do with money has to run through.
“Operating system,” applied to finance, is a borrowed metaphor, not a term with its own body of academic literature — it imports the computing sense of a foundational layer that every application depends on, to make a claim about scope: that a financial arrangement isn’t one tool among many but the substrate other financial activity gets built on top of. This is an honest coinage, not a term of art; it’s included here because the essay leans on it directly, not because there’s a citable definition to point to.
The claim smuggled in by the metaphor is worth naming: calling something an “operating system” implies it is more fundamental, more general-purpose, and harder to replace than a competing “app” — a framing device, not a technical classification.