We should be clear that we recognise the difference between oseteopathy and cranial osteopathy (perhaps better known as craniosacral therapy).
Both have has dozens of research studies published but have tended to be very poor.
Osteopathy has demonstrated decent but not extraordinary effectiveness for chronic lower back pain for example.
Cranial osteopathy is a different matter.
Practitioners of this have contented themselves with very poor quality work, which is no easier or cheaper to do than using better methodology.
But without that level of investigation, no-one can know if it works.
An individual is very vulnerable to a bunch of cognitive biases which lead any anecdote to be unreliable.
It is nearly impossible for any individual to account for:
Natural history
Regression to the mean
Hawthorne effect
Memory liabilities
Experimenter/observer bias
Placebo effect
Thus, trials need to be done.
Before we have evidence it's probably unethical to make claims a therapy.
And IMO thoroughly unethical to profit from them.
Especially when prior plausibility is so low
Southerland and Upledger thought that bones in the skull has a natural repository rhythm and that gentle rubbing could improve this to avoid or cure illness.
This is in defiance of human anatomy and physiology.
No reason to think it might work.
No evidence that says it does.