That makes sense. As your brain shuts down, the finer, higher-level tasks are abandoned.
Right, and a good example of the opposite might be Mars, with some interesting data points in the article below. Mars has very little free oxygen today, but it may have had much more, and perhaps life, billions of years ago, back when it was warmer, had a thicker atmosphere, and had large amounts of liquid water. The oxygen may have been partially or entirely created abiotically, such as from ionizing solar radiation breaking up water vapour in the atmosphere. Regardless, whether life never existed or whether it died out, one theory is that the absence of it caused the oxygen to react with various substances and disappear from the atmosphere, forming things like iron oxides, manganese oxide minerals, and CO2 and CO. The old idea that Mars is a dead, rusted planet may in fact have merit.
Finding abundant free oxygen on an exoplanet along with water may not be a conclusive signature of life, but it would be an incredibly exciting find.
Are you sure it was oxygen they were taking hits of and then saw sparkly things? 
I vaguely recall reading about the same problem in other enclosed spaces like grain bins on farms. Giant bin full of organic material, even when mostly empty since access is a small door, and sealed to help keep mice out, the risk is there.
Yes.
That page lists a few types of manufactured devices that produce oxygen. These means that in theory a naturally occurring chemical reactor could produce atmospheric oxygen, analogous to how natural nuclear fission reactors are believed to have produced nuclear chain reactions without human intervention. So atmospheric oxygen on an exoplanet isn’t an absolute proof of life. Having said that, O2 is so reactive that it seems unlikely that abiogenic mechanisms could produce high concentrations that persist on long timescales. For that to happen on earth, we needed photosynthesis on a massive scale in order to tap into the kind of power required to free up (and sustain) huge amounts of O2.
So while no O2 doesn’t necessarily mean no life, I think substantial concentrations of O2 indicate a strong likelihood of at least photosynthetic life.
Already pretty well answered at this point: our bodies have no mechanism to detect low O2 concentrations, so the first sign of trouble is altered consciousness, maybe even a bit of euphoria, followed very quickly by loss of consciousness. Workers who need to enter confined spaces are typically required to take specialized confined spaces entry training in order to recognize and stave off hazards like this. Every now and then you hear of a worker who entered a confined space and collapsed due to lack of O2 (or presence of non-odorous toxic gases like CO) - and a coworker who went in to rescue the first guy, himself then also falling unconsciousness. In some cases even a third person gets killed trying to rescue the first two.
Military pilots undergo training in high-altitude chambers to learn to recognize the symptoms of hypoxia. They lower the pressure in the chamber to simulate something like 25,000 feet, then have the trainees remove their oxygen masks and execute a series of tasks that demand only minor cognition. Before long they can’t function. Here’s Destin Sandlin, of the SmarterEveryDay channel, in a hypobaric chamber at a simulated 25,000 feet. The video is cued to the moment he takes his O2 mask off:
Within a couple of minutes his pulse ox is down in the 60s. A couple of minutes longer, and he’s having serious problems with cognition and dexterity. 30 seconds later he’s unable to comprehend and follow instructions to put his O2 mask back on, at which point the attendant takes care of this for him.
For your added amusement, here’s a 747 cargo pilot who had no idea he’s hypoxic until controllers figured out what was going on and instructed him to descend to a safe altitude. Article includes full audio; he sounds drunk the entire time, until he descends and recovers normal cognitive function and speech capacity.
Often grain bins and sawdust silos are intentionally kept under a nitrogen atmosphere to prevent fire.