Optimal Page Replacement Algorithm - If more than one frame will not be used again, which one gets replaced?
The optimal algorithm uses whatever is optimal (or one of the optimal possibilities, in case there are several), i.e. whatever produces the least page faults. There isn't necessarily any simpler description of its behavior, though there might be. In general, it depends on the problem; for this specific problem, I don't know whether a simple description exists, though there is probably a simple algorithm that finds an optimal sequence.

FIFO, most used and LRU are heuristics that are useful for the online version; it's not necessarily true that they can be used in any shape or form to obtain optimal page replacement sequences (but you might be able to describe rules for obtaining an optimal sequence using them; I just don't know).