Nitrogen is apparently pretty dangerous too because, from what I understand, your body doesn't feel like it's not "getting enough air" like it does with high CO2 concentrations? Something like that, I don't recall the specifics...
It's build up of co2 in your lungs/body that gives you the urge to breathe, yeah. So any messing around with the air composition could kill you without noticing the lack of o2, as long as you keep breathing thus getting rid of the co2.
