Final answer:
Sterile mineral oil is added to only one tube in the OF test to create an anaerobic environment and test the ability of bacteria to ferment glucose in the absence of oxygen.
Step-by-step explanation:
In the OF test (oxidation-fermentation), sterile mineral oil is added to only one tube to create an anaerobic (oxygen-free) environment. This is done to test the ability of bacteria to ferment glucose in the absence of oxygen. By adding oil to one tube, the microorganisms in it will perform fermentation, while the other tube without oil will allow aerobic respiration to occur.