[d0mine]

You're right. It calls 'void foo(Object o);'

    import junit.framework.TestCase;
    
    public class TestMultimethods extends TestCase {
        public void test() {
          Object test = new Integer(4);                                        
          X x = new X();                                                       
          assertEquals(x.foo(test), "foo(Object)");
        }
    }
    
    class X {
        String foo(Object i) { return "foo(Object)"; }
        String foo(Integer i) { return "foo(Integer)"; }
    }