What is the value of a black onyx ring marked 14KP Dason?

The P is more than likely standing for Plumb, 14 K plumb, not plated. It means that the gold content is exactly 14K. No more, no less. Most gold jewelry, although marked a certain karat, is off, either over or under, just a little. You ought to pay the 5 bucks it'll cost you at your local jewelers to have them test it for content. It could mean the difference of a low selling price and a high selling price.