Reproducibility advances science and de-risks engineering. In ocean wave energy, developing wave energy converters (WECs) requires model test experiments, but there is limited knowledge on the consistency of results if an experiment is reproduced in multiple wave basin laboratories. To better understand reproducibility in WEC experiments, in particular laboratory effects, we reproduced a 1:30 scale model experiment of a case study WEC in two laboratories. This paper compares results between laboratories and evaluates whether, or the degree to which, each experimental parameter contributed to laboratory effects. Performance assessment tests were conducted in intermediate-shallow water regular waves of a bottom-fixed oscillating-water-column (OWC) WEC with a nonlinear, unidirectional flow power take-off (PTO). Despite conducting the experiments according to international guidelines, we found significant differences (15–30%) between laboratories in incident waves and capture width ratio, due to parameters associated with the test environment (wave generation and calibration, and ambient conditions of the air) and the model (deployment position, and PTO modelling). Wave-WEC nonlinear interactions likely amplified the interlaboratory differences. In conclusion, laboratory effects can be significant and, therefore, should be accounted for in WEC model test experiments, especially when there are nonlinearities. We close with recommendations to address laboratory effects relevant for WECs.