When oxygen is present, it is used to produce the molecules such as NADH which are used to produce a proton gradient. With lower pH/more protons in the intermembrane space, such a gradient would allow for ATP production without oxygen.
Work Step by Step
In Fig. 9.15, see that a large number of protons in the intermembrane space is part of the proton gradient used to drive ATP production. This can happen whether or not oxygen is used to produce the proton gradient.