A company holding an (EASA Member State) AOC must have:
1. An "Accountable Manager" who has corporate authority for ensuring that all the operations - including maintenance - can be financed and carried out to the standard required by the AOC - including Part M. This is usually the operator's CEO, but for a small firm may the aircraft owner.
2. A person or group of persons nominated with the responsibility of ensuring that the organization is always in compliance with Part M requirements. Such persons are ultimately responsible to the Accountable Manager. This group is referred to as the "Continuing Airworthiness Management Organization" (or CAMO for short).
3. A "Nominated Postholder" designated by the Accountable Manager who is responsible for the management and supervision of continuing airworthiness activities. This person heads the CAMO. For the small operator this job may be sub-conracted - but not to the maintenance organization that maintains the aircraft.
The Nominated Postholder may not be employed by a separate Part 145 maintenance organization that is contracted to perform maintenance for the operator. (although other members of the "group of persons" may, in some circumstances, be employed by the contractor - as Rigga says the rules are like vapour).