Little is known about mechanical processes of alveolar tissue during mechanical ventilation. Optical coherence tomography (OCT) as a three-dimensional and high-resolution imaging modality can be used to visualize subpleural alveoli during artificial ventilation. The quality of OCT images can be increased by matching the refractive index inside the alveoli to the one of tissue via liquid-filling. Thereby, scattering loss can be decreased and higher penetration depth and tissue contrast can be achieved. We show the liquid-filling of alveolar structures verified by optical coherence tomography and intravital microscopy (IVM) and the advantages of index matching for OCT imaging of subpleural alveoli in a mouse model using a custom-made liquid ventilator.