Not quite. OO messages are flat (a method + a number of arguments), while a DSL messages can have a very complex semantics (even Turing-complete, if necessary).

Not quite. OO messages are flat (a method + a number of arguments), while a DSL messages can have a very complex semantics (even Turing-complete, if necessary). What is common with the Kay view of OO (and different from all the modern OO languages) here is that messages are asynchronous, and this is a very important aspect.