> I guess that I should add that I plan to use a 'natural' stain for the > oak.....
What color is "Natural"?
Do you mean a Clear finish and no stain?
Back to your first question, Most Oaks have a pretty elaborate grain pattern to start with. IMHO you wand to add a wood that is more sedate so that there is not too much grain conflict. That said, Peruvian Walnut or Black Walnut with consistent coloring and grain would go pretty well with Oak. IMHO the table center should be more calm and let the perimeter be more exotic. I would not use Oak in the center. Maple, Ash, or Cherry would be my choice for the center. A more tranquil center will help the game pieces show up more easily.
Other woods that go well together would be
Maple and Walnut Maple and Cherry Walnut and Cherry Walnut and Zebra