Air Operators Certificate (AOC) holder is the operator who are certified by Director-General of Civil Aviation (DGCA) [i.e CAD] to fly for the purpose of public transport.
According to HKCAD webpage:
An aircraft registered in Hong Kong may not fly for the purpose of public transport unless the operator holds an Air Operators Certificate (AOC) granted by Director-General of Civil Aviation. For the grant of an AOC the Director-General of Civil Aviation will take into consideration of the applicant’s previous conduct and experience, his equipment, organisation, staffing, maintenance and other arrangements. Details of the requirements leading to the grant of the AOC are contained in the publication CAD 360, “AOC – Requirements Document”. It could be downloaded at http://www.cad.gov.hk/english/publications_cad360.html.
By Nov 2019, there are 9 AOC holders (i.e 9 Operators) registered in Hong Kong.