OpenLayers does not have the ability to re-project data itself, so all data included in a map must also be provided in Spherical Mercator projection.

Map projection in cartography is a way to present a two-dimensional curved surface on a plane, such as a normal map. Figure 2 shows a Traverse Mercator projection.

The map you're used to seeing pinned on classroom walls and in Atlases is known as a Mercator projection, and was first presented by the Flemish geographer Gerardus Mercator back in 1569.