Air gun--sonobuoy precision echo recorder data acquired on the Ontong Java Plateau and the Manihiki High Plateau demonstrate that these features have upper crustal seismic velocity structures strikingly similar to those observed in deep ocean basins. Each of the crustal layers of the oceanic plateaus, however, tends to be proportionally thicker than its ocean basin counterparts. Our measurements show that the crustal layers of the Ontong Java Plateau and the Manihiki Plateau are 5.0 and 3.1 times thicker than ocean basin crust, respectively. The proportionality also holds for other ocean plateaus, such as the Shatsky Rise, where the structure reported by Den et al. (1969) is 2.7 times thicker than ocean basin crustal layers. Layer 2 A on the three plateaus is poorly predicted by this model. The oceanic plateaus considered in this paper have essentially the same mean crustal density as the much thinner ocean basin crust. These observations, in addition to the oceanic affinities and apparent age progression of the upper crustal rocks of the Ontong Java Plateau, lead us to conclude that this region, as well as most other oceanic plateaus, probably formed as an integral part of the lithosphere at an accreting plate margin.