This is a question we get asked a lot. So what is the difference?
The reality is there are a number of other facts to include here in answering this question such as where in the world you are for starters. Also down to who are you asking, which governing body? Is it the NHS website choices (UK national healthcare system) or World Health Organisation (WHO). Is in the General regulator body of that profession, and of which country? They all vary.
Osteopathy is not a regulated profession in the New Zealand, but is in the UK. Osteopaths must first get a medical license in the US before going on to study osteopathy. Chiropractic is regulated profession in USA, Australia, NZ, and the UK, yet in Switzerland again you have to have a medical qualification before you can specialise in Chiropractic.
So for simplicity in this blog lets review the UK (where we are based).
For myself I find hat Chiropractors and Osteopaths have more differences than similarities in the United Kingdom (UK). Chiropractic at its heart maybe historically focused around the correct and maximal functioning of the nerve system whereas osteopathy (as we were taught in a chiropractic college, so there could be bias) was historically focused on the pooling of fluids in the joints. In either case Chiropractors and Osteopaths are taught how to adjustment or move the joints of the spine with what is recognised commonly as a grade 5 mobilisation that can result in a popping sound. This technique is not standard in in the degree undertaken by physiotherapists, which can do after doing additional courses.
All three professions are regulated in the UK. Osteopathy regulation came in 2000. Chiropractic shortly after in 2001. Physiotherapy is also regulated. There are far fewer chiropractors and osteopaths than physiotherapists. Many Physiotherapists work within the NHS (being referred to by a GP many times). This may also account for why we commonly see people referred to physio through there GP's. The irony is that studies such as the UK Beam trails (research) 1991 and 1994 published in the British Medical Journal (BMJ) showed that spinal manipulative techniques for acute low back pain of less than 3 months duration (these techniques are more commonly performed more by chiropractors and osteopaths) had moderate improvement over regular orthodox medical care. Yet by numbers of available practitioners alone accessibility to these services are limited and may not even be offered by the local primary care trust. To further complicate matters most chiropractors reside in private practice where they have further autonomy and control of patient care, thus limiting patient access to these practitioners. Many chiropractors therefore reply on referral from their existing patients and marketing (which I personally argue is important as results = returning happy patients).
Chiropractors and Osteopaths, like Medical doctors, currently have their own autonomy. Physiotherapists are must register with the Health & Care Professionals Council (HCPC) which also includes arts therapists, biomedical scientists, chiropodists / podiatrists, clinical scientists, dietitians, hearing aid dispensers, occupational therapists, operating department practitioners, orthoptists, paramedics, physiotherapists, practitioner psychologists, prosthetists / orthotists, radiographers, social workers in England and speech and language therapists.
Its a numbers game:
*Number of Chiropractors in Dec 2016 there were 2,781 practicing chiropractors in the United Kingdom
http://www.gcc-uk.org/UserFiles/Docs/Registrations/Report%20on%20the%202016%20registration%20year.pdf
*Number of Osteopaths indicated in Jan 2018 on the Osteopathic Councils website was "over 5,200"
http://www.osteopathy.org.uk/training-and-registration/
*Number of physiotherapists activated registered practicing in 2017 was 27,500 and has for the last 10 years remain between 25,000-28,000. 54,000 are currently registered.
https://www.statista.com/statistics/318906/numbers-of-physiotherapists-in-the-uk/
Note: To practice as either a Chiropractor, Osteopath or Physiotheratist in the UK with out being registered with the appropriate government register is illegal