The german federal association of med. doctors, in a meta-study in 2009 ordered a scientific inquiry into osteopathy1. First, as you state there is no clear definition pertaining to osteopathy everywhere. In short, osteopathic methods can be applied without the philosophy riding on it in the U.S. If you remove that cruft, it is striking that many methods are similar to those of so-called "manual therapy". This is why european doctors rather use that label, and it's specifically those methods which are supported by some evidence, first and foremost in the field of chronic spinal pain syndromes.
Reference: Bundesärztekammer: "Wissenschaftliche Bewertung osteopathischer Verfahren". In: Dtsch Arztebl 2009; 106(46): A-2325 / B-1997 / C-1941
The paper first goes into depth about the history, also in the U.S., the definition problems, and the economic forces of the profession. The history of osteopathy associations is mentioned and the UK compared with the US, after which follows a description of the German situation.
In III.2 the central question is posed, is osteopathy Heilkunst, i.e. does it heal people? This would have legal consequences. It is stated that that question is equivalent to if it's an effective method without side effects. First attempts to show this are listed. The density of evidnce-based studies with that subject is found low.
With section IV, the main investigation starts and separates the (in the US) associated philosophy from the healing method. The difficulty that osteopath diagnostics differ from medical standards and cannot be categorized using the ICD is mentioned. IV.2 finally has the core result, coming from the work of two experts, that all osteopathy methods with evidence based study support are also part of "manual therapy": one of the reports found 62 relevant papers using Medline of which 16 were of evidence class Ia, 16 of Ib, and the rest II-IV and meta-studies. The second report stated that most relevant literature isn't even listed in Medline.
Basically, there are few illnesses where we have reliable statements about the effectivity of the respective osteopathic method, (mainly chronic pain syndromes of the spine), some RCTs (randomized controlled trials) also pointing at visceral effects, apart from the musculo-sceletal field. Studies about "craniosacral" effects are methodically flawed, the effects remain speculative. Even osteopathic papers have shown that humans cannot "feel" movements of those skull joints.
About risks and side effects, there is nothing like post-marketing surveillance in osteopathy. So, precondition is a thorough medical investigation and differential diagnosis, and important are written after-reports, also for legal reasons. The experts found no reports of problems.
(I'm not a native english speaker, please bear with me)