To reconstruct subsurface water conditions using deep-dwelling planktonic foraminifera, we must fully understand how the oxygen isotope signal incorporates into their shell. We report δ¹⁸O in four species sampled in the eastern North Atlantic with plankton tows. We assess the size and crust effect on the isotopic δ¹⁸O and compared them with predictions from two equations. We reveal different patterns of calcite addition with depth, highlighting the need to perform species-specific calibrations.