The objective of the present work is to investigate the ultra-high temperature (1273–1473 K) plastic deformation behavior of a nano oxide dispersion strengthened Fe-18Cr ferritic steel (n-ODS-18Cr steel) over a range of strain rates (10 −2 to 10 s −1 ). The flow (true) stress-true strain behavior of this steel, reported for the first time, has been utilized to obtain the flow stress as a function of temperature and strain rate. From the above data, the strain rate sensitivity parameter (m) and also the activation volume have been obtained as a function of temperature (1273–1473 K). It is then demonstrated that the flow stress values obtained in the present steel over the temperature range 1273–1473 K is due to a single dominant strengthening mechanism, i.e. Orowan dispersion strengthening. Hence, the predictions of the theoretical models which assume Orowan strengthening as the dominant mechanism can be compared with the experimental flow stress data. Such a comparison indicates that the model due to Rosler and Arzt predicts the experimental data very well while the prediction of the model due to Brandes, Kovarik, Miller, Daehn and Mills is satisfactory. It is also shown that the experimentally obtained activation volume is consistent with the dislocation detachment model. © 2019 Acta Materialia Inc.