What's the proper way to make an Ocaml subclass with additional methods?

In Ocaml I'm struggling with subclassing and types:

class super =  
  object (self)  
  method doIt =   
    ...  
end;  

class sub =  
  object (self)  
  inherit super  
  method doIt =   
    ...  
    self#somethingElse  
    ...  

  method somethingElse =  
    ...  
end;  

let myFunction (s:super) =  
  ...  

myFunction new sub

Apparently in Ocaml, class sub is not a "subtype" of class super, because the sub#doIt method calls a method in sub that is not present in super. However, this seems like a pretty common use-case for OO programming. What is the recommended way to accomplish this?