Starting in 1949 Berlin was split not just between occupying powers, but between two states. The western half became part of the Federal Republic of Germany (aka West Germany) with the establishment of that state in May 1949, while the eastern half became part of the German Democratic Republic (aka East Germany) with the establishment of that state in October 1949. The currency in Berlin would depend on what side of the city you were on, and initially both sides of the city did accept both currencies, with harder divides coming with the establishment of the Berlin Wall in 1961.

Prior to that there was much more of a free flow of people between both sides of the city, with citizens of each side working, and more importantly shopping, on the other side. Payment would occur in the currency of the employer with there being a preference among citizens for the internationally accepted Westmark. Additionally citizens of both sides had incentives to shop on the other side, with East Berliners going West for luxury goods and West Berliners going East for heavily subsidized food. After the closure of free movement in 1961 both marks were more solidified in their respective areas as the Westmark became harder to obtain by average citizens in the East.

It is an important side note that the government of the German Democratic Republic (GDR) sought Westmarks as they were far more internationally accepted than Ostmarks were, and they served as a hard currency that the state could use in international trade. This was, however, not something that the state encouraged of its citizens. While citizens of the GDR did have access to Westmarks through travel to the West, and family connections, the state did monetarily try to limit the influence of the Westmark.

This was first done in 1957 when GDR authorities announced that all Ostmarks printed after 1948 were now being replaced with new Ostmarks. This action allowed for East German citizens to exchange their old Ostmarks for new ones, but the old Ostmarks held by West German, and West Berlin, businesses and authorities were functionally worthless after that. The reform served to internally bolster the Ostmark, while discouraging trade for Ostmarks on the part of Western businesses. It is important to note that outside of Eastern Bloc countries the Ostmark had no real international value prior to the international recognition of the GDR in 1971.

In sum, from 1948 to 1957 both marks freely moved throughout Berlin and were used in both sides of the city. After 1957 the movement of Ostmarks was more limited, and after 1961 it became more difficult for East Germans to obtain Westmarks, while non-internationally accepted Ostmarks became far less valuable to Western businesses with little movement of people from East to West on a daily basis. The establishment of the Berlin Wall kept this in place through 1989.