I believe irreversible declining oxygen levels are a possibility. Anything is possible, so there's no reason to say this could not possibly happen. I do not think we are at risk of this happening any time soon, so we shouldn't get shook up about it. I've never heard about this being a possible problem in the near future.
As it currently stands, oxygen levels in the ocean are decreasing and one of the causes is global climate change. It's no secret that we've mistreated the environment, and this appears to be yet another consequence of it. I believe that, unless we enact some serious changes in how we run our lives on a global scale, trouble is only going to get worse until we can't ignore it anymore.
I do not think that irreversibly declining oxygen levels are a possibility. I think that there will always be a way to reverse the damage that may be done to oxygen levels around the world. All it takes is a matter of countries and government funding more plants in public places and policy changes.
No, irreversibly declining oxygen levels are not a possibility, because oxygen levels are always something that we can make more of. Plants are things that we can grow. If we need more oxygen, we can just make more plants. Oxygen is a renewable resource. We will never run out completely.