This document provides a clear, step-by-step introduction to calculating and interpreting Odds Ratios (OR) using R. Using a practical health science scenario (Smoking and Lung Cancer), it demonstrates how to build a 2x2 contingency table, extract individual group odds, and replicate the results using both the epitools package and logistic regression (glm).It is specifically designed for beginners, featuring plain-English explanations of data framing, factor levels, and how to avoid common coding layout traps like inverted risk estimates.