If you need to ask the question then I would suggest you have a lot of heavy reading and headache coming your way if your serious, but thats not to say it can't be done. The following is assuming you are talking about in JAR land(?).
AOC - Definitely if you want to do commercial aerial work (charter, photography etc...), any the requirements to be found suitable for holding and AOC are pretty stringent (and documented in JAR-OPS3, below). If however you want to undertake solely Flight instruction however - then no need for an AOC as that's not charter work.
http://www.jaat.eu/publications/jars/606970.pdf (JAR-OPS3) would be a good starting point.
Also the type of AOC required is dependant on the aircraft you intend to fly, (correct me if i'm wrong) but I believe there is a watered down "Single engine piston AOC" if you fancy using the R22/R44 etc..
This is my limited understanding on the issue.
Aucky