I think the OE tours have more flexibility you can customize the tour a bit with your group
The regular tours you are on a tighter schedule with a larger group
description of OE tours
Designed for travelers who prefer an added measure of intimacy and flexibility, our new Oceania Exclusive tours limit group size to just 16 guests maximum. The Oceania Exclusive format is available on specially selected shore excursions in nearly every port we visit, around the world.