This study aimed to validate the effectiveness, accuracy, and feasibility of the cut-plane method for measuring the side branch (SB) ostium area in three-dimensional optical coherence tomography (3D-OCT) pullbacks performed in the main branch (MB). A total of 109 sets of OCT pullbacks from the MB and SB of coronary artery bifurcation lesions were analyzed using Vivolight OCT software. Measurements of the SB ostium area from the MB and SB pullbacks were analyzed. Measurements of the SB ostium area from the actual SB pullback were used as a reference. 3D cut-plane analysis was used to estimate the correlations and mean errors with the reference measurements. Thirty-four sets of OCT images from the C7XR system and 75 sets from the CornarisTM system were analyzed using Vivolight software. There was a strong correlation between the reference measurements of the SB ostium area and the measurements obtained through 3D cut-plane analysis in the overall dataset (r = 0.925). This correlation was observed consistently with both the C7XR system (r = 0.955) and CornarisTM system (r = 0.900). Similar results were found in subset analyses of true and nontrue bifurcations (r = 0.936; r = 0.898, respectively) and in left main (LM) or non-LM bifurcation subsets (r = 0.932; r = 0.873, respectively). There were strong correlations between measurements of the SB ostium area by 3D-OCT and the reference measurements, and thus may be a reliable and accurate alternative to direct OCT pullback examinations of the SB.