Background: To protect employees against the hazards they may be exposed to during work, employers must provide preventive occupational safety and health (OSH) in international and national legislation. Employers can provide these services by their firm or by third parties. Structures created outside the workplace, within the scope of the private sector, are called joint health and safety units (JHSUs). The number of studies examining the effectiveness and adequacy of JHSUs is limited. Material and Methods: This study experimentally and hypothetically reveals the effectiveness level of JHSUs in providing OSH by conducting an extensive field study with a survey of 381 health and safety professionals and developing an Occupational Safety and Health Services Effectiveness Scale. Moreover, it compares those who receive the service from JHSUs and those who receive the service from the workplace, and the effectiveness level of JHSUs is evaluated. Results: The fit index as a result of confirmatory factor analysis with the scale is at an acceptable level with χ^²/df = 3.18, RMSEA = 0.076, TLI = 0.89, and CFI = 0.91. Cronbach’s α values of the factors are at a high level of reliability with PMCE = 0.949, OSHA = 0.927, OSHCM = 0.875, OSHEC = 0.869, OSHSQ = 0.877, OSHSE = 0.852. Conclusions: The effectiveness of JHSUs in OSH services differs from the effectiveness achieved by internally assigning an occupational safety expert. The Student’s t-test accepts all hypotheses H1–H6 regarding the developed scale and that there is a significant difference between the external and internal groups in all factors. Med Pr Work Health Saf. 2025;76(1)