This type of thing was experimented with during the second world war on aircraft engines. Pure oxygen is too reactive. It's inclined to explode from the heat when it contacts the inlet valve. Plus the even bigger problem is that it just burns too hot. The pistons would melt in no time.
Think, our atmosphere is (roughly) 23% oxygen, and 75% nitrogen (Which is inert) you'd need 4X the amount of fuel to maintain stoich.
On the other hand, Nitrous oxide is only 66% oxygen, and 33% nitrogen. Which is an oxygen richer mix, but not as bad as pure oxygen. However with most wet nitrous kits extra fuel is also added.
The previous comments about gas consumption is right. On a big snotty engine a ten pound bottle is quickly used.