NO. There is no evidence that wearing a mask can reduce oxygen levels.
Even doctors and healthcare professionals wearing N95 masks (which fit very tightly around the face and are made to create a seal around the edge of the mask) are not at risk of hypoxia. However, for any person with preexisting lung or breathing problems in general, they should speak with their doctors about their concerns regarding masks.
Masks offer protection to you by filtering out virus particles. Also, wearing a mask protects others from you when you cough, sneeze, talk, or just breathe. In addition to physical distancing and frequent hand washing, masks are recommended to prevent the spread of COVID-19. Ensure you use a mask that allows you to breathe comfortably while talking and walking and that fits well on your face.