British geologist and seismologist who discovered evidence for the existence of the Earth's core.

After training at the Royal School of Mines, Oldham joined the Geological Survey of India in 1879, eventually serving as superintendent (until 1903). His study of the Assam earthquake in 1897 revealed three separate kinds of earthquake waves on seismographic records and established that these waves traveled along different paths and at different speeds through the Earth. He also documented for the first time the vertical displacement of strata as a result of earthquake motion. To explain that earthquake waves that propagated directly through the Earth traveled slower than waves that passed through obliquely, Oldham hypothesized a core that slowed the waves observed 180° from the earthquake epicentre.