ENT's and OMFS overlap most on trauma. Other than that, it depends on what they each like to do. I've seen OMFS's do septoplasties and I know ENT's who do BiMax advancement.
Either specialty should only practice within the scope of their training. So I, as an ENT, do not do odontogenic cyst removals when in the mandible. An OMFS who didn't get properly trained in septoplasties shouldn't try to do those.