I took DD to an osteo when she was 4 months old after a suggestion from the MCHN. DD had a pretty decent flat spot on her head and I wanted to try and avoid any serious issues if I could. Her head shape changed very quickly. She was also a spewy bub and for a few days after her osteo appt she would be relatively spew free. It didn't get rid of it completely though for her.
The osteo was lovely and had I known more when DS was little, I would have had him checked over aswell.