• Oxygen's role in cellular respiration and ATP production is crucial, but excessive oxygen leads to reactive oxygen species (ROS) formation. Evidence suggests that excessive ROS may result in pulmonary dysfunction. • High fractions of inspired oxygen can lead to the development of absorption atelectasis. The clinical relevance of this phenomenon in veterinary is unclear. • Although the decision to use high versus low oxygen levels perioperatively should be done on a case-by-case basis, it is likely that in most cases, use of a high fraction of inspired oxygen is not warranted.