Overall Difficulty has no effect in osu!catch besides the maximum score achievable for that difficulty. For that reason, we have the following
Ranking Criteria guideline:
Overall Difficulty should have the same value as the Approach Rate. This is just a standardized value, as Overall Difficulty does not affect gameplay nor the amount of fruits a spinner has, just the maximum score of a difficulty.
However, if a difficulty uses a lower Approach Rate than one or more difficulties in the difficulty level below it, then the Overall Difficulty should be equal to that of the highest Overall Difficulty value in the previous difficulty level.
Basically, setting the OD = AR of the difficulty is the best solution in most cases.