Since Standard ML doesn't directly support OO, I imagine the language is simpler. Some high-level comparisons on the MLton website: http://mlton.org/OCaml
Thanks for the pointer. My first thought is that I didn't see anything overwhelmingly compelling there, although I can see the benefits of having a "standard" language with many implementations vs. an implementation defined language.
As far as OCaml OO goes, I rarely use it (with one exception: objects implementing the "visitor pattern" for traversing the AST of a C program [1]) so its existence doesn't affect me. Perhaps I'd view the resulting language complexity as a negative if I had to maintain OCaml code that was OO heavy.
EDIT: Matt Might's summary on SML and OCaml is also quite good http://matt.might.net/articles/best-programming-languages/