The family Nairoviridae consists of a group of tick-borne viruses in the order Bunyavirales. Several nairoviruses have been demonstrated to cause human disease, the most notable being Crimean-Congo hemorrhagic fever virus that can have fatality rates greater than 30%. Encoded in the nairovirus genome is an ovarian tumor domain protease (OTU) that reverses posttranslational modifications of proteins by ubiquitin (Ub) and the Ub-like protein interferon (IFN)-stimulated gene product 15 (ISG15). This activity of the OTU and proteases from other viruses as deubiquitinases/deISGylases has been connected with suppression of the type I IFN response, a key part of early cellular responses to viral infections. As a result, they have been proposed to be virulence factors and are considered potential therapeutic targets. Interestingly, it has been observed that OTUs from different nairoviruses do not possess the same activity or relative preference for Ub and ISG15. This raises the prospect that these viruses may not engage the immune response in the same way. Additionally, ISG15 shows significant interspecies diversity that has been shown to impact interactions with viral proteins and potentially host tropism. Regrettably, prior investigations of OTUs were only able to characterize the activity of a few nairoviruses, leaving it unclear on how substrate specificity may differ across the family. This work addresses this gap in knowledge through structural and biochemical approaches to understand the impact of virus and host diversity on nairovirus-host interactions. OTUs from diverse nairoviruses representing the whole virus family were broadly assessed for their activity against Ub and 12 species' ISG15. This revealed that DUB and deISGylase activity is predominantly restricted to a few, closely related virus lineages, and that ISG15 preference generally correlates with known host associations. Six novel OTU structures were solved by X-ray crystallography, revealing the impact of sequence diversity on structural features. Combined with mutational analysis, this revealed the molecular drivers for interaction with Ub and ISG15, including the ability to shift preferences of one substrate versus another. Overall, this work provides a foundation to develop tools to further probe the role of the OTU during infection and its potential as a therapeutic target.