During the Hirnantian Age (Late Ordovician) the Oslo Region was located in a subtropical setting with siliciclastic input and carbonate production. At that time the sea level fluctuated in the Oslo Region during three regressive-transgressive episodes, some of which involved subaerial exposure and coastal valley erosion. The last major sea-level drop resulted in the formation of a conspicuous network of incised valleys that were subsequently filled with sediment during the transgression in the latest part of the Hirnantian. The continuing transgressive event rapidly flooded the exposed land areas in the central Oslo–Asker district. The areas towards the west in mainland Asker and Sylling in the adjacent Modum district were first transgressed in the late Rhuddanian (Early Silurian). Primarily eustatic processes affected the area, but synsedimentary faulting may also have been in play. There are two distinct palaeovalley trends: one at Hovedøya in Oslo more or less NW–SE, with narrow valley sides, the other at Kalvøya and surrounding areas trending approximately NE–SW, with one valley more than 10 km long. There may have been more than one filling phase. Sediment fill of the last-formed incised valleys were mapped and correlated across a large area of the Oslo–Asker district. Four new members of the Langøyene Formation are proposed: the Skaueren, Høyerholmen, Pilodden and Kalvøya members. Strata of the lateral Langåra Formation began deposition during the Katian. The rest of the formation as well as the Skaueren and Høyerholmen members contain fossils belonging to the cold-water Kosov fauna of early to mid Hirnantian age. The Pilodden and fossiliferous parts of the Kalvøya members include a mix of cold-water Kosov and warm-water Edgewood faunas of late Hirnantian age. Detailed descriptions of many incised valley fill sections in Oslo and Asker are included to show the spectrum of the sediment fill composition.