If Patterson believes that oxygen is removed by the enzyme and only N2 is
produced. What happens to the oxygen? Where do you/he think it goes?
Do you think it stays in the enzyme?

Not enzyme. Bacteria. Yes, that's exactly what happens. The oxygen is
consumed by the bacteria.

This might help your confusion:
When you state that oxygen is consumed by the bacteria in the
denitrification process where does it end up? where does it go? what form
does it take?

If your answer is not H2O you need to do more reading.