Randomization will be performed by the coordinating centre (Univ. of Milano-Bicocca, Italy) by randomly pre-generated lists; the coordinating centre, without informing the blind-rater, will communicate to the local Osteopathic Clinic these details. Patients will be given instructions to start treatment (OMT vs. SHAM in blind) that will be administered by a treating D.O. unblinded to treatment allocation. Patients will receive 4 weekly treatments completing the headache diary along the whole study (up to T120). The blind-rater will review diaries relative to the each previous period and ask patients to complete the HDI/HIT-6 at 30 days (T30, end of treatment), 60 days (T60, follow-up-1), 90 days (T90, follow-up-2) and 120 days (T120, end of study) with respect to randomization (T0). An osteopathic structural examination will be performed again at T120 (end of study) for GAS scoring. A central database will be filled.