One of the main scientific objectives of the Gravity field and steady state Ocean Circulation Explorer (GOCE) gravity field satellite mission is its contribution to the global unification of height systems. In this study, we compute the offsets of three height datums in North America (NAVD88, CGVD28 and Nov07) against a common equipotential surface. NAVD88 and CGVD28 are the official vertical datums for the USA and Canada, respectively. Nov07 is the latest unofficial adjustment of the first-order levelling network in Canada. This datum is only used for the validation of geoid models. The offset for each datum is determined from a combination of ellipsoidal, orthometric and geoid heights. The ellipsoidal heights on benchmarks come from the GNSS networks of Canada and the USA while geoid heights are computed from currently the best GOCE-based geopotential model in North America, i.e., go_cons_gcf_2_tim_r3. The orthometric heights of the GNSS stations are available from the adjustments of the vertical control networks of both countries. Among the various factors that contribute to the uncertainty of the computed datum offset, we investigate the effect of the omission error of the GOCE geoid by means of the EGM2008 model. In Canada, where GNSS/levelling stations are irregularly distributed over the landmass, the effect of the GOCE omission error on the computed offsets reaches one decimetre. Due to the much more densely distributed GNSS/levelling stations in the USA, the effect of the GOCE omission error on the offset of NAVD88 is 3 cm. Therefore, the effect of the omission error of GOCE-based geopotential models should be taken into account in the height datum unification on the North American continent if we aim at the one centimetre accuracy.