ill just throw on a bit more, on one of my horns I never had the O-rings so eventually all the metal banging made the end of my slide deform a bit (no big deal as the repair man fixed it no problem) However any music store that sells brass cleaning kits and what not ought to have the O-rings that will fit your horn, most of the O-rings will fit all trumpets unless its something super custom and special etc. Anyways when I got my Strad my former lessons teacher told me that he didnt want to see the slides get messed up, so he threw on some O-rings he had. Therefore in my experience the O-ring serves two purposes, 1) to dampen the sound of the big metal clang to a thud, 2) to keep the end of the slide from deforming as quickly as it would without it. Just drop by your local music shop is what I would say, its not hard, just be careful I was putting on the O-rings by myself my first time they snapped alot, so buy quite a few (they're cheap) and try not to snap them, you'll get it after a bit. Have fun!