You can, but then you don't get any of what OOP actually offers. Message passing isn't the same thing as dynamic dispatch. OOP is a very different paradigm.

I think you are both unknowingly talking past each other: my understanding is that Smalltalk-style "object-oriented programming" ("everything is a message!") is quite distinct from C++/C#/Java/Rust "object-oriented programming" ("my structs have methods!")