I offer no advice as a "gun expert" in regards to finishing grips or stocks, my advice comes from 30 years as a cabinetmaker and master finisher.
The poly/stain mixes...word of caution, they are usually tinted with dyes that are not very lightfast and the color will fade. (Major importance on a piece of furniture, prolly not so much on a gun kept in a pocket or safe). The first coat will raise the grain leaving it feeling rough. A quick once over with some 320 paper will get rid of this but you will need another coat. Another coat will then darken the color as will all subsequent coats. If you want to go this route then I would ever so slightly dampen the surface with water, to raise the grain, then the 320, then your color coat. If you are happy with the color then any added coats should be clear.
As far as keeping the Holly white, all coatings, ALL will yellow over time. Almost all woods will darken as well. Woods like holly and maple yellow. Coatings that are least prone to yellowing usually have substantial amounts of UV inhibitors in them. These don't last though, one reason sunscreen has to be reapplied to be effective. Again this all plays into wood exposed to sunlight and the elements not going to be such a rapid effect on guns I would think. I can think of several dozen coatings that would be far superior on grips and stocks to anything you would get at the local hardware store. Problem is these coatings are usually not available to the general public and require a little bit of experience to use with good results.
That being said, tongue oil or some of the wipe on polys would be a good choice for do-it-yourselfers. Tongue oil holds up great, is easily touched up and the sheen can easily adjusted from satins to gloss. It does have a pronounced darkening effect though. If using a poly just look for one that promotes it's non-yellowing ability. It may yellow in a year on your kitchen cabinets but would probably take several years to do so on your grips.
Also make sure whatever coating you use on the finish coat to also apply to the backside as well. Wood, for a lack of better words always breathes. Having both sides coated will even up any moisture absorption and cut down the chances of splitting or warping.
Oh and geez, another thing...on the oak, seal the inner surface of the screw holes well. The acids in the oak can interact with the steel and turn the oak black around the holes. Oh and by the way... there are great ways to stain woods that don't rely on stains but instead chemical reactions with the woods using things like steel wool, or drain cleaner and on and on. Since I could type for the next three weeks (and don't want to) on the subject I would suggest a Google search in you have any interest in it.
