>>a simple oxygen-monitoring probe<<
They are $30 to $40 each and you want them to be disposable?
If the earlier patient's DNA was still on it, it's an indication that it wasn't even WIPED between uses. The minimum should be that it gets a wipe-down with an alcohol swab.
Since the oxygen sensor works by measuring how different frequencies of light are absorbed by tissue, a thin, transparent, disposable cover could be developed for it.