you might need to call your OSC and find out if he can supply us with any details.’ ‘I can’t call Heath,’ said William. ‘He only ever contacts me.’
— Hidden in Plain Sight, Jeffrey Archer
From context, OSC is I think a criminal informant but I can’t find an exact definition?
Google tells me one police-related definition is Office of The Surveillance Commissioners, but that doesn’t work here.
EDIT (in response to comment).
Heath is a drug user who is an informant. William and the person he is talking to are both UK police detectives working in the drug squad.