Thank you for the question. If you mean below sea-level, but still on land (such as in Death Valley), the answer is likely no, the percentage of oxygen in air does not decrease when you go below sea-level. The percentages of different elements in the atmosphere are constant up to about 60 miles above sea level, implying that the percentages are also constant just below sea level. However, I couldn't find any studies of below see-level measurements.

The reason for unchanging composition of air up to 60 miles is "turbulent mixing." There is so much wind that all the air is well-mixed. I would expect this to hold below sea-level as well, where there is still wind.