Sample size: Sample size calculation is based on non-inferiority design. In this study, investigators aim to evaluate if OT is no less effective in terms of retention (the primary outcome) than the gold standard, methadone. Previous clinical trials with methadone's 3- month retention rate as their primary outcome, demonstrated a rate of approximately 80%. Setting the type Ι error of 0.05 and power of 80% with clinically acceptable margin of 10%, the calculated sample size equals 100 participants in each treatment group. Considering a dropout rate of 20%, total sample size will be 240, 120 participants randomized to each group of the study. Statistical analysis plan: Retention in treatment will be compared between two groups using Fisher's exact test considering P value ≤ 0.05 as significant. Secondary outcomes will be compared between two groups using Fisher's exact test, independent samples T-test and One-way ANOVA test where applicable. Prediction of outcome variables will be based on generalized linear model and generalized estimating equations (GEE) model.