When an O-Power of the same kind is used at a higher level or lower level, it would just override the current one. if it is of the same level, the time would just reset.
The most efficient way to grind is a Black Writ of challenge at the Battle Chateau with Exp power lvl 3 and against Audinos while your Pokemon holds a Lucky Egg.
In any case, should you fail to have the money required another way is to trade it to someone else and have them train it for you or challenging the E4 with a lucky egg amd using the switch-out method to get good experience.
Hope I helped!
-It's a secret!