It's French, so you always have to reverse the order of the words (e.g. NATO in French is OTAN). So Operational Amplifier becomes Amplifier Operational (in French words) and that's how you would get to AOP. Crazy French.
Then: why 18-bits? Isn't 16-bits enough? Your system really that stable that you can use those final bits? There are lots of cheap 16-bit ADCs out there, such as the ADS1115. Sample rates of that one are quite low, so that's another factor: what sample rate do you need?
And out of curiosity: where does the 12-bit come from? The regular Arduinos have a 10-bit ADC.