Part M Subpart G lays out the requirements for a CAMO. AOCs have to have one, owners (non-commercial and non-"large") can have one or standalones exist to which AOCs can sub-contract some tasks and non-AOCs can contract entirely.
Have a read of EASA Opinion 02/2008; this outlines some changes that will affect non-commercial, non-large owners.
When I say non-large, I mean <5700 kg or not twin helicopters. I'm confusing myself. Opinion 02/2008 seeks - at behest of UK CAA - to redefine large aircraft as complex motor-powered aircraft.
