In the Norwegian show called "Beforeigners", (which has time travellers that are threated like refugees) there's been quite a few mentions and encounters of 18th century Bohemian gangs selling drugs in the streets of modern day Oslo.
So why is that? Are they native to Kristiania or did they just migrate there in the show's modern day timeline?
Are you sure that the show really deals with the people from the 18th century Christiania, before the dissolution of the union with Denmark, not the 19th century?
Although I don't have access either to the series of Immigrant History (of course, in Norwegian) or the history of the city of Oslo, even for the 19th century, the prospect of identifying such 'Bohemian gang' group does not look promising.
The oldest historical statistic of foreigners (actually the people by nation) of Norway that I can find only dates back to 1865 (Source: Central Statistic Bureau, Norway): https://www.ssb.no/a/histstat/tabeller/3-10.html
The people born in Austria (-Hungary) firstly appeared in the official statistic in the entry of 1875 AD, but the number seems to fairly be small (61), compared, to give an example, with, those who from other areas in 'Germany' (1,471).
In 1875, the population of Oslo commune alone amounted to 76,866 (Source). Even if almost all of these Czech population (maximum ca. 60) were resided in Oslo, I suppose it would be a bit difficult to converge a distinct 'foreign' community only with at most ca. dozen family.