Sorry to bring boredom to an entertaining thread, but I am motivated by spite, since once again the vastly overrated Jaffa Cake has been bigged up, which as a chocolate hob nob man, always nibbles my nuts.
Nobody signs the OSA. Some people sign a document saying they understand the OSA (which is pretty pointless as ignorance of it is not a defence) but the OSA applies to every UK citizen...which kinda completely answers OPs question.
And I knew this a long time before the internet, wiki and pprune were invented.
As I'm sure you all do as well.
Time for a hob nob.