In the present work, we investigate bubble dynamics (bubble growth and collapse) and onset of flow oscillation in a natural circulation boiling loop (NCBL) using a high-speed camera (500 fps) and numerical simulation. The steady-state analysis of the loop flow rate yields the onset of flow oscillations and a marginal stability boundary, which are reasonably close to the experimental results. The simulation based on four-equation drift flux model predicts the amplitude and frequency of experimental flow oscillations. We observe that the growth and collapse of a vapor bubble with a critical diameter of 3.42 mm triggers the flow oscillation in the NCBL. The bubble diameter and shape play a major role in initiating the flow oscillation followed by spiral motion, path oscillation, and shape transition of the non-spherical bubbles. The numerical simulation satisfactorily predicts the flow oscillations.