Prosecutions under that act are very rare and were never intended to protect the political class from the embarrassment that ensues from information about their sexual peccadillos reaching the public domain. Of course, such legislation is the perfect vehicle under which to hide information embarrassing to the government or the royal family and it is widely believed the spirit of the legislation has been abused, frequently in conjunction with the issue of DSMA (formerly just 'D') notices.
There are laws covering libel and slander which might be more relevant in cases such as that concerning Green but as you can probably imagine, it is dream material for the lawyers and the media. The OSA is something of a coverall piece of legislation which, by its invocation, can cause more intrigue than the matter might otherwise merit.
As far as the Damian Green matter is concerned, the policeman's tale and Green's denial of all knowledge of how porn was downloaded to his computer opens up a whole new, and possibly far more dangerous, can of worms. That is, if we are to believe Green's story!
The relevant part of the Act would be this section:
Information resulting from unauthorised disclosures or entrusted in confidence.
Tom