This paper describes a study of the impact of confinement on the hydrodynamic performance of oscillating-foils hydrokinetic turbines (OFHT). This work aims to contribute to the development of standards applying to marine energy converters. These blockage effects have indeed to be taken into account when comparing measurements obtained in flumes, towing tanks, and natural sites. This paper provides appropriate correction formula to do so for OFHT based on computational fluid dynamics (CFD) simulations performed at a Reynolds Number Re = 3 × 106 for reduced frequencies between f* = 0.08 and f* = 0.22 considering area-based blockage ratios ranging from ε = 0.2% to 60%. The need to discriminate between the vertical and horizontal confinement and the impact of the foil position in the channel are also investigated and are shown to be of second-order as compared to the overall blockage level. As expected, it is confirmed that the power extracted by the OFHT increases with the blockage level. It is further observed that for blockage ratio of less than ε = 40%, the power extracted scales linearly with ε. The approach proposed to correlate the performance of the OFHT in different blockage conditions uses the correction proposed by Barnsley and Wellicome and assumes a linear relation between the power extracted and the blockage. This technique is shown to be accurate for most of the practical operating conditions for blockage ratios up to 50%.