Nice idea.
Am sure people will come up with flight suggestions, but what about alternatives that do not require you to fly out of T3:
1. Buy the cheapest one-way fare you can find out of T3 and do not fly it. (I assume you have status and your folks are flying club so you don't need a club flight to get access to lounges).
2. Buy a fully refundable J or F ticket, but again do not use it. Then move your OSL flight of 14th to 13th and stroll to T5 once you have brought your folks to their gate. Get your other flight refunded.