Serial port describes the connection between O2k and Computer. With the USB-Cable 2.0\Type A-B connected, select Serial port in the Connection window. Depending on the O2k series, it is possible to connect with a Serial port or USB port.
Reference: MiPNet22.11 O2k-FluoRespirometer manual
MitoPedia O2k and high-resolution respirometry: DatLab