This paper explores demographic processes behind ethnic geographies in Oslo. We compare data for census tracts in 2001 and 2011, and decompose ethnic composition and segregation on local mobility, national migration, international migration and natural demographic change. The study comprises five national groups: Poles, Somalis, Sri Lankans, Iraqis and Pakistanis, plus aggregates for Nordic and non-Nordic residents. A key observation is that local mobility weakens the status of original settlements without a corresponding effect on levels of ethnic segregation. For several groups, local mobility increases both own-group exposure and separation from the Nordic majority. International migration, in contrast, increases spatial integration between Nordic and non-Nordic residents. Natural change accords with our expectation and strengthens both minority representation in established eastern settlements and ethnic segregation in Oslo at large. Certain features of the Oslo context, e.g. lack of multi-ethic experiences, domination of owner-occupied dwellings and redistributive policies, may explain the surprising results.