A ventilator for ventilating a patient with setpoint values of an oxygen concentration in the respiratory gas, having a programmable control apparatus, a respiratory gas source, an oxygen source, a gas mixing unit, a display and operating elements, as well as at least one interface (8, 18, 28) and an oxygen sensor for determining an oxygen saturation of the patient, wherein the programmable control apparatus is configured to perform the following method: • Specifying a target value or limit values for the SpO2, and • Specifying an FiO2 in the respiratory gas, and • Specifying a closed-loop control period for the ventilation with the FiO2, and • Ventilating with an oxygen concentration in the respiratory gas corresponding to the specification for the time duration of the closed-loop control period • Ascertaining current measured values for SpO2 during the time duration of the closed-loop control period • Showing the measured values for SpO2 on the display, and • Showing the specification for FiO2 on the display, wherein the signal quality of the SpO2 signal is ascertained, for example based on the light intensity, and visualized on the display such that a good signal quality is shown visually differently from a poor signal quality, characterized in that the method includes the ventilation taking place with an oxygen concentration in the respiratory gas corresponding to the specification for the duration of the closed-loop control period and, in the process, measured values of the SpO2 being recorded during the ventilation and it being analyzed whether the measured values of the SpO2 correspond to the target value and, in the case of a defined deviation of the measured values of the SpO2 from the target value, a new setpoint value for FiO2 is calculated and/or applied for the following closed-loop control period and wherein, based on the deviation of the measured values for SpO2 from the target value, a change is calculated for the FiO2 and a symbol is shown on the display, wherein the symbol gives the viewer information about whether the following setpoint value for FiO2 - compared to the current FiO2 value - stays the same or is changed. • Specifying a target value or limit values for the SpO2, and • Specifying an FiO2 in the respiratory gas, and • Specifying a closed-loop control period for the ventilation with the FiO2, and • Ventilating with an oxygen concentration in the respiratory gas corresponding to the specification for the time duration of the closed-loop control period • Ascertaining current measured values for SpO2 during the time duration of the closed-loop control period • Showing the measured values for SpO2 on the display, and • Showing the specification for FiO2 on the display, wherein the signal quality of the SpO2 signal is ascertained, for example based on the light intensity, and visualized on the display such that a good signal quality is shown visually differently from a poor signal quality, characterized in that the method includes the ventilation taking place with an oxygen concentration in the respiratory gas corresponding to the specification for the duration of the closed-loop control period and, in the process, measured values of the SpO2 being recorded during the ventilation and it being analyzed whether the measured values of the SpO2 correspond to the target value and, in the case of a defined deviation of the measured values of the SpO2 from the target value, a new setpoint value for FiO2 is calculated and/or applied for the following closed-loop control period and wherein, based on the deviation of the measured values for SpO2 from the target value, a change is calculated for the FiO2 and a symbol is shown on the display, wherein the symbol gives the viewer information about whether the following setpoint value for FiO2 - compared to the current FiO2 value - stays the same or is changed. wherein the signal quality of the SpO2 signal is ascertained, for example based on the light intensity, and visualized on the display such that a good signal quality is shown visually differently from a poor signal quality, characterized in that the method includes the ventilation taking place with an oxygen concentration in the respiratory gas corresponding to the specification for the duration of the closed-loop control period and, in the process, measured values of the SpO2 being recorded during the ventilation and it being analyzed whether the measured values of the SpO2 correspond to the target value and, in the case of a defined deviation of the measured values of the SpO2 from the target value, a new setpoint value for FiO2 is calculated and/or applied for the following closed-loop control period and wherein, based on the deviation of the measured values for SpO2 from the target value, a change is calculated for the FiO2 and a symbol is shown on the display, wherein the symbol gives the viewer information about whether the following setpoint value for FiO2 - compared to the current FiO2 value - stays the same or is changed.