In the presence of oxygen, many spoilage microbes will produce a pellicle on the surface of the beer. But not all of them. Lacto plantarum often will not produce a pellicle, even when there's lots of oxygen available. And if the headspace is purged with CO2, an active infection generally won't produce a pellicle. So a lack of pellicle is not a good indication for lack of infection.
Also, there are plenty of spoilage microbes that are obligate anaerobes, and will die in the presence of oxygen. Clostridium and megasphaera, for example. Others, like brett, pedio, and lacto are facultative anaerobes, and while they enjoy a high oxygen environment, they do just fine anaerobically.