What would happen to the Earth if the atmosphere suddenly turned to a thick gas similar to honey or oil? Sure, everything would die from lack of oxygen, but what would actually happen?

For the purpose of this question, assume that the atmosphere changed from one second to the next, and without any other changes to the planet - ie. no change in climate. Assume also that the atmosphere changed in the same way all the way up to Karman Line.<br><br> Would there be an immediate change that people would notice, or would something need to actually move in order for us to notice anything?