ORO.FC.100
(e) When engaging the services of flight crew members who are working on a freelance or part-time basis, the operator shall verify that all applicable requirements of this Subpart and the relevant elements of Annex I (Part-FCL) to Regulation (EU) No 1178/2011, including the requirements on recent experience, are complied with, taking into account all services rendered by the flight crew member to other operator(s) to determine in particular:
(1) the total number of aircraft types or variants operated; and
(2) the applicable flight and duty time limitations and rest requirements.
So it is explicitly allowed for in the regulations, and the responsibiliy for everything other than having a current LPC rests with the operator. Once you have completed the OCC then there is no need to do it again as long as you meet all the same currency or renewal requirments as if you were full time.
For part NCC the OCC requirements are not too onerous. Many operators have written into their manuals that they will accept a standard sim recurrent for the ground/flight training, and can accept previous courses for things like CRM & DG. There should be something on the safety equipment of their particular aircraft, but there often isn't.
For AOC, where there is more required including OPCs and line training, it is still theoretically possible but in practice rarely worth the company's investment if you are not going to be full time. Most cases of freelancing for AOCs involve people who were previously full time and just need to be kept current.