Oceanic crust is the uppermost layer of an oceanic tectonic plate. The crust overlies the solidified and uppermost layer of the mantle. The crust and the solid mantle layer together comprise an oceanic plate.The plates of the Earth together are referred to as the lithosphere.ceanic crust is significantly simpler than continental crust and generally can be divided in three layers. 1  Layer 1 is on an average 0.4 km thick. 2  It consists of unconsolidated or semiconsolidated sediments, usually thin or even not present near the mid-ocean ridges but thickens farther away from the ridge.