Filling the grain will have no significant effect on how well the doors are protected. It's purely cosmetic at this stage. If you decide you want to fill the grain, get some paste filler from a woodworking supply, add some PIGMENTED stain (ground pigments, not dye stain) to match the color of your oak, wipe it on, wipe it off across the grain after it sets up for a while, let it dry, then do it again until grain is filled to your satisfaction. After final application of paste filler is dry, give it a lite wet sanding with 360 or 400, then more wipe-on poly. Don't use dye stains or universal colorants because they will fade under exposure to sunlight. Pigmented stains are forever.
