How do doctors keep the blood oxygenated during open heart surgery?

They use a Heart-Lung machine which takes over for the body's organs. It's also called a cardiopulmonary bypass machine. It takes over pumping blood while the heart is stopped and also takes care of adding oxygen to the blood, thus doing the work of the heart and lungs. An early version of the heart-lung machine was constructed as long ago as 1885 in Germany. Reliability improved markedly when heparin was discovered in 1916. Heparin is used to prevent blood from coagulating.

https://www.cedars-sinai.org/health-library/diseases-and-conditions/s/surgery-of-the-heart.html
https://en.wikipedia.org/wiki/Cardiopulmonary_bypass