THECORBYLOON, I'd be the first to agree that not every civil servant is subject to the control of the OSA. However, those that do sign acceptance of the act do fall under its control. There are a surprising number of them about and they include people who don't necessarily work for MI5/MI6.
I've worked alongside scientists from DEFRA, the MRC and other government organisations who have signed the act. Many staff in government agencies are also subject to the act for reasons not necessarily to do with information security.
Besides, in this day and age, the OSA is not the only restriction in preventing a civil servant from disclosing information on their role within an organisation. You only have to look at a Civil Service employment contract to come to this conclusion.