Oxen - castrated male cattle (Bos taurus) - were used as draught animals in Finland, Estonia and Sweden for centuries. From the medieval period onwards, the wider phenomenon of oxen culture is evidenced by zooarchaeological material and historical documents, which reflect, among other things, the castration age. When males were castrated at a young age - a method used in Sweden, according to the ethnological sources - it affected the growth of the metacarpal bones; thus, the metacarpals of these oxen should be distinct from those of cows and bulls. However, when males were castrated as adults - a method used in Estonia - it had less effect on the bones, so the metacarpals of these oxen are more likely to resemble those of bulls. Documents from the early modern period indicate that in Finland, cattle were castrated as calves (as they were in Sweden), but some medieval bone finds indicate that they were castrated in adulthood (as they were in Estonia). To resolve this apparent contradiction between the zooarchaeological data and the written evidence, and to understand the cultural transitions in farming practices relating to draught oxen in the northern Baltic Sea region, we studied morphometric changes in 621 metacarpals in Finland and Estonia dating from the medieval and post-medieval periods (13th to 20th century) and compared them with previously published contemporaneous data from Sweden. To investigate the size and shape of the individuals and to distinguish oxen from bulls and cows, we applied widely used osteometric criteria, including the maximum breadth of the distal epiphysis and Mennerich indices. For a set of uncertain samples from medieval Turku, Finland, we identified the sex by amplifying the TSPY gene and X and Y chromosome-specific variations in the ancient DNA. Our results confirm that throughout the medieval and post-medieval periods, late castration was practised in Estonia and early castration was practised in Sweden. In Finland, however, we detected a temporal change in the castration age: cattle were castrated as adults in the medieval period and at a young age in the post-medieval period.