Death from oxygen deprivation seems to occur with a sliver left on the bar, and instantly.
Instead, once the bar reaches its last pixel and empties, it would be good if your character turned blue and lost health quickly while the bar is completely clear. A few seconds grace to close the airlock or swig a can of air for those who have otherwise remained in good shape.
The fact that the blue would be "in your face" could only help make it more obvious you're dying ;)