Those levers take quite a lot to open, plus sliding the door would also take a lot of strength as well. I'm not sure a child that old has what it takes to unlock and open those doors, but I guess it's possible.
I was told they have some type of childproof device they can lend you to insure the door cannot be opened. Not sure exactly what it is and I've not personally seen one, but this was passed on to me by one of our clients who had a small child.
I have seen the kind where you insert a small rod in the top frame, I'm just not sure if it was on the Oasis-class ship or not.
Have your agent call RCCL and inquire about this for you.
Travel Agent/Cruise Specialist w/13 yrs exp and 48 Cruises on 11 cruise lines! Favorites: Paul Gauguin - Tahiti: Uniworld River Cruises - Europe; Celebrity Solstice-class ships; Holland America - 12-nights Baltics & Russia; RCCL - 14-nights Greek Isles, Turkey, & Croatia; Holland America - 14-day Alaskan cruisetour; 10-night Canada/New England cruise; 21 days Hawaii w/7-night NCL cruise; Oceania - 25 days in Asia; more than 3 months touring Europe by train. And many all-inclusive resorts!