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The Oligocene is a geological epoch within the Palaeogene, between the Eocene and Miocene, approximately 34 to 23 million years before the present. It is also the corresponding interval of rock strata in chronostratigraphic classification.

The boundaries are defined using reference sections and correlation criteria, while their numerical ages are calibrated by dating and can be refined. A rock is therefore not assigned to an epoch solely because a rounded age estimate falls inside a convenient numerical interval. Fossils, stratigraphic relationships and independent dating must be interpreted together.^[ref]

The Palaeogene is the geological period from 66.00 to 23.04 million years ago in the consulted chart. It contains the Paleocene, Eocene and Oligocene, preceding the Neogene within the Cenozoic.^[ref]