But isn't Oak an open pored wood? It's susceptible to moisture and mold.
Will depend in part on the meaning of open pored.
Of the many species of Oak, they are divided by the lumber industry into 2 general categories, Red Oak and White.
The distinction between them is that Red is "ring porous" and White is not.
With a ring porous oak (Red) you can actually use it like a straw. Not across the grain (rings), but in the direction of the grain. Looking at the end grain with a magnifier, one can see holes in the red oak but not in white. The holes typically run through to the other end of the piece.
But oak in general is called an open-grain wood, even with the finest sanding or a sharp scraper the surface will always have some divots, uneven-ness. Wood finishers often apply a grain filler prior to a final finish. Filling grain can keep dust, etc. Out of the small crevices.
Maybe this open grain is what leads to the impression that oak will get moldy?
Maybe the Red is more susceptible?
White Oak is considered to be somewhat better suited for exterior use...
Sorry to run on, but I had a career with wood. I always remember it as 'Red= Ring Porous". The R's.