It still has classes, and implementation inheritance with method overloading, but they're completely decoupled from subtyping (so e.g. subclasses are not necessarily subtypes - a class can use self-type in an argument position). And types themselves are structural, with row variables and polymorphism. If you need to define an equivalent of a named interface, you just define a type alias, like any other. And when you define a class, you get an auto-generated type alias for the structural type of objects that it will instantiate. [1]

End result actually looks surprisingly familiar to someone coming from C++ or Java, at the first glance. It's really a shame that it's not something that OCaml is famous for; it deserves to be. I wish I could have an object model that expressive in C#.

[1] https://caml.inria.fr/pub/docs/manual-ocaml/objectexamples.h...