I would probably recommend combining the OSC and Time commands to accomplish your goal.
The OSC command reads like:
OSC 000:00/000:00/000:00 Then ON (OFF/ON/OFF think of it like wait/on/off for the time it is allowed to operate)
Then use the if Time command to set the period the OSC command is allowed to be on:
IF Time 00:00 to 00:00 Then ON
Remember the Apex reads top to bottom so write it stacked something like:
Fallback OFF
Set OFF
OSC 000:00/000:00/000:00 Then ON
IF Time 00:00 to 00:00 Then ON
Calculate however long the OSC command needs to be on to dose your H202 amount and then add as many If Time commands as you need. Remember the If Time command will need to be for as long as your OSC command needs to run.
I am sure someone may come along with a simpler way, but since it seems like you are wanting a varied schedule with 3 times a day and then every 15 minutes from 12-6am I can only think of this as the simplest way.