Laboratory tests such as the erythrocyte sedimentation rate (ESR), C-reactive protein (CRP) and Immunoglobulin M (IgM) have been used as markers of inflammation and disease activity in rheumatoid arthritis (RA), although there is still no clear consensus on when to use one, the other, or all. We aimed to evaluate the level of OPG in RA patients compared with control in order to estimate the predictive value of OPG. Eighty five patients with active RA, attending rheumatology department at Al-Sadder General Hospital in the province of Maysan-Iraq, diagnosed according American College of Rheumatology (ACR) revised criteria were included. The patients' tender and swollen joint counts were calculated. Laboratory investigations were done including ESR by Westergren method, CRP, IgM and OPG by ELISA method, assessment of disease activity using DAS28 score. A non-significant change (p>0.05) was seen in the level of BMI, significantly increased (p<0.01) in levels of ESR, CRP, IgM, and OPG as compared to the control group. All patients showed disease activity at the time of the study, their DAS28 scores ranged from 5.20 to7.40 (Mean ±SD 6.43±0.80). There was a positive and highly significant (p< 0.01) correlation between DAS28 values and ESR, CRP, IgM, and OPG values (r-value was 0.662, 0.695, 0.689, and 0.636, respectively). The OPG was more specific than Rf (IgM, CRP, and ESR) in the diagnosis of RA. So it could be a useful assay in establishing the diagnosis of RA, especially in ambiguous cases or RF negative patients with RA, and also a better predictor of disease severity.