High-fidelity flow boiling simulations are conducted in a vertical mini channel with offset strip fins (OSF) using R113 as a working fluid. Finite-element code PHASTA coupled with level set method for interface capturing is employed to model multiple sequential bubble nucleation using transient three-dimensional approach. The code performance is validated against experiments for a single nucleation site in a vertical rectangular channel. To assess code performance, a study on the bubble departure from the wall in a mini channel with OSF is carried out first. Contributions from the microlayer are not considered due to low heat flux values applied to the channel (1 kW/m2). The influence of surface characteristics, such as contact angle and liquid superheat on bubble dynamics, is also analyzed as well as the local two-phase heat transfer coefficient. For higher void fractions, two conical nucleation cavities are introduced in the same channel with OSF. Observed bubble characteristics (departure diameter, bubble departure frequency) are evaluated and bubble trajectories are presented and analyzed. The local heat transfer coefficient is then evaluated for each simulation. The results show approximately a 2.5 time increase in the local heat transfer coefficient when the individual bubbles approach the wall. With smaller bubble nucleation diameters, the heat transfer coefficient can increase by up to a factor of two. Thus, the current work demonstrates the flow modeling capability of the boiling phenomena in complex geometry with OSF.