Abstract Purpose To compare a new swept source optical coherence tomography (SS-OCT) based biometer (OA-2000) with the IOLMaster v5.4 (partial coherence interferometry) and Aladdin (optical low-coherence interferometry) biometers in terms of axial length measurement and failure rate in eyes with cataract. Design Reliability study Methods A total of 377 eyes of 210 patients were scanned with the three biometers in a random order. For each biometer, the number of unobtainable axial length measurements was recorded and grouped as per the type and severity of cataract based on the Lens Opacities Classification System III by the same experienced ophthalmologist. The Bland-Altman limits of agreement (LoA) method was used to assess the agreement in axial length measurements between the three biometers. Results The failure rate was 0 eyes (0%) with the OA-2000, 136 eyes (36.07%) with the IOLMaster and 51 eyes (13.53%) with the Aladdin. Chi-squared analyses indicated a significant difference in failure rate between all three devices (P<0.001). Logistic regression analysis highlighted a statistically significant trend of higher failure rates with increasing severity of nuclear, cortical and posterior subcapsular cataracts. Bland-Altman statistics indicated small mean differences and narrow LoA (OA-2000 versus IOLMaster -0.09 to 0.08mm; OA-2000 versus Aladdin -0.10 to 0.07mm; IOLMaster versus Aladdin -0.05 to 0.04mm). Conclusions The OA-2000, a new SS-OCT based biometer, outperformed both the IOLMaster and Aladdin biometers in very advanced cataracts of various morphologies. The use of SS-OCT technology may be the reason for the improved performance of the OA-2000 and may lead to this technology becoming the gold standard for the measurement of axial length.