Trying to perform numeric operations on a cell array is usually pointlessly complicated, so the first thing to do is to put that numeric data into a numeric array. Then your task is trivially easy:
>> V = [A{2,2,:}]
V =
5 4 3 2 1
>> [val,idx] = min(V)
val = 1
idx = 5
>> A(:,:,idx)
ans =
'O' 'P'
'Q' 1
Read this to understand how the first line of code works:
