We report on the morphological tagging of Old Serbian in the Universal Dependencies framework. To facilitate the manual annotation, we pre-processed the data with the Old Church Slavonic 2.12 UDPipe model. The decision was based on the known similarity of these two languages as well as on the declared performance of this model compared to other models for historical varieties of Slavic languages. With over 3,000 manually annotated tokens, we evaluated the performance of the relevant pre-trained UDPipe2 models of historical Slavic languages. Besides, we also trained and evaluated custom models with UDPipe1 containing the annotated Old Serbian data. We have found that: (1) for this particular domain and amount of training data, the most suitable model is UD Old East Slavic – Birchbark 2.12, although its declared performance is much lower than that of Old Church Slavonic; (2) even 3,000 tokens of Old Serbian increase the performance of UDPipe1 models almost to the level of the Birchbark 2.12 model.