In terms of possible contamination, it is hard to test unless you have done a forced fermentation test on that particular wort and your attenuation is higher than the forced fermentation result. If you have some of the yeast cake from that fermentation, you can do a culture and check for possible other species growing aside from yeasts. If it is a wild yeast, it would be a bit difficult to distinguish but some common contaminants such as pediococcus can be distinguished by the culture and cell size pretty easily in a cell plate.
The Oxygen theory is interesting. From what I remember, the presence of too much O2 leads to a lack of expression of yeast character. With all that oxygen, the yeasts would produce more sterols, stronger cell walls and thus more efficient reproduction. This over production of yeasts takes away acetyl-CoA used for the Krebs cycle thus slowing down ester formation and other flavor by-products. Thus, more yeast cells usually results in that loss of yeast character. This would suppress ester production from the WY1318 and produce a cleaner product, but not necessarily increase the attenuation from the yeast.
It may also result in a higher level of fusel alcohols due to the lack of acetyl-CoA and incomplete breakdown of the sugars as the yeasts use the oxidation of the fusel alcohols to create more NADH to support their metabolic cycle.
Everything in my library states that yeast attenuation is highly linked to the wort fermentability.
