Hi Larry,
Unfortunately there aren't any baggage lockers that I know of at St. Michel-Notre Dame station. The large train stations in Paris, such as
Gare du Nord, definitely do have baggage lockers for use. A large locker would fit about 4 medium sized roller suitcases for 9,50€.
If you have exactly 6 hours between flight arrival CDG and take off at ORY, my guess is you'd have about 2 hours of free time. Landing at CDG, passing immigration, collecting your bags, getting to the train station, buying tickets, waiting for the train, taking train into Paris will take about 1.5-2 hours. Traveling from central Paris to ORY by train/bus + finding your check-in will take about 45-60 min. I you have to arrive at ORY 1 hour before your flight, that leaves you with about 2 hours.
Perhaps a couple hours at a cafe near Notre Dame (of which there are many) wouldn't be so bad with some luggage.
Best regards,
Ben