An Organised Crime Group (OCG) is a group of three or more individuals acting together to commit serious crimes for financial or material gain, often involving complex structures, planning, and exploitation, with activities ranging from drug trafficking and fraud to cybercrime, using violence and intimidation to further their goals.

Under the UK Serious Crime Act 2015 (legislation.gov.uk) (Section 45), an OCG involves three or more people acting together to further criminal activities, with the purpose of engaging in such activities.
In essence, OCGs are well-organized criminal enterprises, distinct from individual criminals, that operate on a continuous basis to achieve significant criminal objectives.