Hi Sniperdon227
I remember this question and the mark scheme is awful! I have actually queried this question with the exam board and forced a response. The answer I would give to this question is this:
Pt I H2 I H2O II O2 I OH- I Pt
The reason for this is because from the equations given H2 is oxidised to H2O (zero to +1) and then in the next equation O2 is reduced to OH- (zero to -2).
I hope that helps a bit, and as I have already said, the answer I have given would gain the marks, confirmed by AQA.
Thanks
Alex