The world is as follows: each observation x_i is one of "the mind can choose between A and B", "the mind can choose between B and C" or "the mind can choose between C and A" (conveniently encoded as 1, 2 and 3). Independently of any past observations (x_1 and the like) and actions (x_1 and the like), each of these three options is equally likely. This fully specifies a possible world, no?

The mind, then, is as follows: if the last observation is 1 ("A and B"), output "A"; if the last observation is 2 ("B and C"), output "B"; if the last observation is 3 ("C and A"), output "C". This fully specifies a possible (deterministic, computable) decision procedure, no? (1)

I argue that there is no assignment to U("A"), U("B") and U("C") that causes an O-maximizer to produce the same output as the algorithm above. Conversely, there are assignments to U("1A"), U("1B"), ..., U("3C") that cause the O-maximizer to output the same decisions as the above algorithm, but then we have encoded our decision algorithm into the U function used by the O-maximizer (which has its own issues, see my previous post.)

(1) Actually, the definition requires the mind to output something before receiving input. That is a technical detail that can be safely ignored; alternatively, just always output "A" before receiving input.