Absolutely and I am basing my assumptions that RAF "mess stewards" fulfilling the current role will be bound by the Forces official secrets act. Whether or not ths can be applied to civillians I do not know, there is also the question of partnering long term.
The OSA applies to all. Whether one has signed it or not.
As if it needed pointing out, mess stewards are far less likely to come into contact with classified information than say an ATCO or a civvy bench tech in station workshops.