Earlier this year, my Open Skies flight from ORY was cancelled. I'd advise you to have the relevant number for BA on your phone and enough cash to get from ORY to CDG if you need to get across there quickly.

As you might imagine, there can be an awful lot of people standing line to be rerouted when a flight is cancelled. I stood in line with everyone else but rang BA at the same time. They sorted me out on the phone long before I got anywhere near the front of the line. In my case, I had to make a quick dash to CDG. I kept my taxi receipts and phone bill and BA very quickly reimbursed me those out-of-pocket expenses.