How much does a low oxygen saturation level affect judgment?

Depends how low. Down to about 80% there isn't much effect at all at least for short periods of time. Longer periods weeks to months can have cognitive effects. Below 80% saturation there can be clear effects on judgment even for brief periods. The lower the saturation the bigger the effect.
Oxygen. A low oxygen saturation definitely affects judgment. Your brain is one of the organs that is heavily dependent on proper oxygenation. If you decrease the oxygen saturation, judgment may be impaired.