There are several things that could decrease the oxgen in our atmosphere. 1. If we cut down all the forests of the world, which some people refer to as the earth’s “lungs”; 2. If we have a huge number of volcanic erruptions that filled the sky with debris and reduced the sunlight reaching our plants and forests so that they could no longer turn carbon dioxide into oxygen; and 3. If a large asteroid hit the earth, throwing up enough dust and debris to block out the sunlight for a while, having a similar effect as 2. None of these things are very likely, as we would hope that humans would stop deforestation as soon as they realised that the oxygen in the atmosphere was decreasing, and the other two options (volcano and asteroid) are very rare events that have happened to the earth in the past, but the earth has always recovered afterwards.
I don’t think we’ll ever produce enough carbon dioxide to prevent us having enough oxygen.. Even if we burn another 1000 billion tons of fossil fuels, the amount of oxygen in the atmosphere would only be reduced by about 20%. The other effects this would have would be more dangerous than oxygen depletion itself: like hotter temperatures and increased pollution.