Venovenous extracorporeal membrane oxygenation (VV ECMO) relies on effective blood drainage, and cannula design plays a crucial role in optimizing blood oxygenation. Recirculation, where oxygenated blood returns to the drainage cannula without systemic circulation, can negatively impact ECMO efficacy. It was hypothesized that drainage cannulae with a longer side hole portion would reduce recirculation and improve blood oxygenation. This in vitro study investigated the impact of the length of the side hole portion and the intervals of the side holes on blood oxygenation in a simulated VV ECMO circuit. Five cannulae were prepared with the following side holes: (a) 1-cm intervals from the tip to 5 cm; (b) 1-cm intervals from the tip to 10 cm; (c) 1-cm intervals from the tip to 15 cm; (d) 3-cm intervals from the tip to 15 cm; and e) 5-cm intervals from the tip to 15 cm. With blood drained from the right atrium and returned to the superior vena cava, recirculation rates and PaO₂ at 150 s after ECMO initiation (descending PaO₂ order) were: (c) 32 ± 1% and 160 ± 4 mmHg; (d) 35 ± 1% and 149 ± 5 mmHg; (b) 38 ± 1% and 139 ± 5 mmHg; (e) 39 ± 1% and 133 ± 4 mmHg; and (a) 57 ± 3% and 94 ± 7 mmHg. These findings suggest that cannulae with a longer side hole portion may optimize blood oxygenation by reducing recirculation. Furthermore, shorter side hole portions on drainage cannulae are not recommended for VV ECMO.