Objective: To evaluate the effects of variation in ambient oxygen pressure on orthodontic tooth movement in guinea pigs. Material and Methods: Seventy-two guinea pigs randomly distributed into two groups (A and B) were evaluated in the study. All specimens were fitted with orthodontic appliances to distalize maxillary incisors. Group A was controlled under conditions of oxygen pressures at sea level (150 masl, 157 mm Hg) and Group B under conditions of oxygen pressures at altitude (3405 masl, 107 mm Hg). The clinical (distance between the distal-incisal angles of the maxillary incisors), biochemical (serum alkaline phosphatase), and histopathological characteristics (osteoblast and osteocyte count) were evaluated before placing the orthodontic devices and after 24 and 72 hours. Results: In the clinical evaluation, the distance between the distal-incisal angles of the maxillary incisors, on day one and three, was significantly higher in group B compared to group A (p=0.002 and p=0.001, respectively). In the biochemical evaluation, the level of serum alkaline phosphatase on the first and third days was significantly higher in group B compared to group A (p=0.001 and p=0.001, respectively). In the histopathological evaluation, the osteoblasts and osteocytes count on day one and three was significantly higher in group B compared to group A (p<0.05). Conclusion: Oxygen pressure at high altitude positively influenced orthodontic tooth movement in guinea pigs, improving its clinical, biochemical, and histopathological characteristics.