Background: Over the past years, the introduction of direct-acting antivirals (DAAs) revolutionized chronic hepatitis C treatment. We aimed to characterize and assess treatment efficacy in three specific groups of patients treated with DAAs: those with active solid malignant tumors (SMTs), hematological diseases (HDs) and hepatocellular carcinomas (HCCs). Methods: A total of 203 patients with active oncological disease (SMT n = 61, HD = 67, HCC n = 74) during DAA treatment in 2015–2020 selected from the EpiTer-2 database were analyzed retrospectively and compared to 12,983 patients without any active malignancy. Results: Extrahepatic symptoms were more frequent in HD patients (17.2% vs. SMT = 10.3%, HCC = 8.2%, without = 7.8%, p = 0.004). HCC patients characterized with the highest ALT activity (81 IU/L vs. SMT = 59.5 IU/L, HD = 52 IU/L, without = 58 IU/L, p = 0.001) more often had F4 fibrosis as well (86.11% vs. SMT = 23.3%, HD = 28.8%, controls = 24.4%, p = 0.001). A significant majority of subjects in HCC, HD and SMT populations completed the full treatment plan (HCC = 91%; n = 67, HD = 97%; n = 65, SMT = 100%; n = 62). Concerning the treatment efficacy, the overall sustained virologic response, excluding non-virologic failures, was reported in 93.6% HD, 90.16% SMT and 80.6% in HCC patients. Conclusions: As presented in our study, DAA therapy has proven to be highly effective and safe in patients with active SMTs and HDs. However, therapy discontinuations resulting from liver disease progression remain to be the major concern in HCC patients.