I'd assume that the reg key could be used with any OS version of Opera,
but YMMV.

   For one day only, you can get an ad-free version of Opera.

    From Download.com, (and for one day only, valid from 12 a.m.
Tuesday, August 30      to 12 a.m. Wednesday, August 31 2005 (PDT)).

    Simply e-mail registerme@opera.com to obtain a registration code
Normally $39USD.