Globally, national immunization programs are at risk of disruption due to severe health system constraints caused by the ongoing Corona Virus Disease 2019 (COVID-19) pandemic. To assess the impact of COVID-19 on the availability of supplies of routine childhood immunization in the Oromia Regional State in Ethiopia. A health facility-based mixed-methods of study design was conducted. Both quantitative and qualitative data were collected using pre-tested questionnaires and key informant interview question guides, respectively. Descriptive statistics such as frequencies, means, and standard deviations were performed. Binary logistic regression analysis was employed to assess the associations between the COVID-19 pandemic and the availability of vaccine-related supplies at health facilities in study area. The qualitative data were analyzed using a thematic content analysis approach. Of the total 448 study participants, 214 (47.8%) reported that COVID-19 has disrupted childhood vaccines availability. A significant proportion of participants agreed with the disruption of BCG (62.1%), OPV (48%), IPV (40.4%), and PENTA (36.9%) vaccine availabilities. These findings were also supported with key informant interviews. Fear of not maintaining physical distance (71.4%) followed by government lockdown (68.1%) and inadequate supply by local providers/suppliers (62.4%) were the most frequently reported likely causes for the observed disruptions. A large proportion of participants (87.7% and 81.7%) reported disruptions in the supply of face masks and hand gloves, respectively. As to the binary logistic regression analysis, the study participants from hospitals were 1.72 (1.01, 2.68; 95% CI) times more likely to report the impacts of COVID-19. Corona Virus Disease -19 significantly disrupted the availability of supplies of childhood immunization in the Oromia region. The most disrupted vaccines and related supplies were BCG, OPV, IPV, PENTA, facemasks, and hand gloves. An effective vaccine supply management is crucial to prevent disruptions during pandemics such as COVID-19.