Is it possible for someone to od on a usual 1 line of snorted cocaine?

All is possible: Most Cocaine is 20-30% in purity, some even lower than that. If someone used to 10% purity gets some stuff that is 90% purity, that could constitute a excessive amount of Cocaine for them. In addition, Cocaine can be "cut" with other things, including pcp (phencyclidine) which can cause an od-like reaction. Maybe you should consider this a wake-up call and work on getting sober. Get help.