Background Out-of-hospital cardiac arrest (OHCA) remains one of the leading causes of death worldwide, and bystander CPR with public-access defibrillation improves OHCA survival outcomes. The COVID-19 pandemic has posed many challenges for emergency medical services (EMS), including the suggestion of compression-only resuscitation and recommendations for complete personal protective equipment, which have created operational difficulties and prolonged response time. However, the risk factors affecting OHCA outcomes during the pandemic are poorly defined. This study aimed to assess the characteristics and outcomes of OHCA patients before and during the COVID-19 pandemic in Thailand. Methods This single-center, retrospective cohort study used data from electronic medical records and EMS paper records. All OHCA patients who visited Ramathibodi Hospital’s emergency department before COVID-19 (March 2018 to December 2019) and during COVID-19 (March 2020-December 2021) were identified, and the number of emergency department returns of spontaneous circulation (ED-ROSC) and characteristics in OHCA patients before and during the COVID-19 pandemic in Thailand were collected. Results A total of 136 patients were included (78 men [59.1%]; mean [SD] age, 67.9 [18] years); 60 of these were during the COVID-19 period (beginning March 2020), and 76 were before the COVID-19 period. The overall baseline characteristics that differed significantly between the two groups were bystander witness and mode of chest compression (p-values < 0.001 and < 0.001, respectively). The ED ROSC during the COVID-19 period was significantly lower than before the COVID-19 period (26.67% vs. 46.05%, adjusted OR 0.21, p-value < 0.001). There were significant differences in survival to admission between the COVID-19 period and before (25.00% and 40.79%, adjusted OR 0.26, p-value 0.005). However, 30-day survivals were not significantly different (3.3% during the COVID-19 period and 10.53% before the COVID-19 period). Conclusions During the COVID-19 pandemic in Thailand, ED ROSC and survival to admission in out-of-hospital cardiac arrest patients were significantly reduced. Additionally, the witness responses and mode of chest compression were very different between the two groups. Trial registration This trial was retrospectively registered on 7 December 2021 in the Thai Clinical Trial Registry, identification number TCTR20211207006.