The worldwide standard albeit guidelines for compliance are the (updated) ICAO Annexes to the Chicago Convention 1944 in particular Annex 6 'Operation of Aircraft'.
These have been in place for many years and are adopted by all signatories to the Convention however each individual country may file 'Differences' which are detailed in addendum to the appropriate Annex.
The question of differing standards of implementation, execution, supervision and safety oversight however is a very wide one - there are in my experience differences between many JAR European countries, that's even before we start to contemplate other continents
An AOC can be issued limited to certain geographic regions or 'world wide', it can limit aircraft weight or will detail specific aircraft type in larger weight bracket, it can be 'Passenger' or 'Cargo' or both...