The early Holocene marine faunas of the Oslo area were studied in classical works by Brøgger (1901) and others, but there has been relatively little research in recent decades. Classical fossil localities near the marine limit at ca. 220m above sea level around Oslo are now difficult to localize, overgrown or completely lost and it is difficult to collect new material. However, the material of Brøgger, Øyen and others can be studied in the extensive Quaternary collections of the Natural History Museum in Oslo. The historical locality in Skådalen at the marine limit, new datings of Mytilus edulis 10,260 ± 70 14C years and Balanus 6200 ± 40 14C years were done in this thesis. Both samples seemingly from the same locality but the ages do not correspond. Holtedahl (1960) encountered the same problem when dating the same locality. For the Sognsvann locality Mytilus edulis were dated 10,150 ± 70 14C years, corresponding well with the timeline created in this thesis, both Sognsvann and Skådalen samples are seemingly collected outside the main ridges of the Aker substage. Also, Vardeåsen, Nesodden locality, Mya were dated 9915 ± 50 14C years. CT scanning, isotope profiling (ẟ18O) and LA-ICPMS analysis were performed on Mytilus edulis from the Skådalen and Sognsvann locality showing evidence of freshwater input and seasonal changes in Ba values correspond well with seasonal changes in ẟ18O. SEM imaging of ostracods from the Sognsvann location made it possible to identify species and further enhance the possibility of freshwater input, placing the locality close to the ice front. The Preboreal Oscillation is suggested to be related to either the Ås – Ski substage or the Aker substage, the connection with the Aker substage being more likely.