The evaluation of safety performance in occupational health and safety assessment series (OHSAS) 18001-certified companies provides useful information about the quality of the management system. A certified organization should employ an adequate level of safety management and a positive safety culture to achieve a satisfactory safety performance. The present study conducted in six manufacturing companies: three OHSAS 18001-certified, and three non-certified to assess occupational health and safety (OHS) as well as OHSAS 18001 practices. The certified companies had a better OHS practices compared with the non-certified companies. The certified companies slightly differed in OHS and OHSAS 18001 practices and one of the certified companies had the highest activity rates for both practices. The results indicated that the implemented management systems have not developed and been maintained appropriately in the certified companies. The in-depth analysis of the collected evidence revealed shortcomings in safety culture improvement in the certified companies. This study highlights the importance of safety culture to continuously improve the quality of OHSAS 18001 and to properly perform OHS/OHSAS 18001 practices in the certified companies.