The Obliterator Set is the armor set you will receive by participating in RvR. The boots and gloves are fairly easy to obtain, but if you don't start early, the chest can be elusive. The boots are a bind on equip drop that can be looted from enemy players, including in scenarios. This is one of few pieces that you can find on the Auction House, so check in periodically if you have trouble getting them to drop. The gloves become available at Renown Rank 17 and can be bought from Renown Gear vendors in the top level of ORvR keeps. The chestpiece drops from the chest at the end of a successful keep siege. Only a gold bag will yield your Obliterator chestpiece, and figuring out contribution is sometimes confusing and arbitrary. The best way to get this chestpiece is to simply participate in a lot of sieges – eventually, if you play well, chance will favor you. This set does have a rank requirement to wear: all the gear requires rank 17 to wear, the boots require RR 15, the gloves RR 16 and the chest RR 17. The two piece set bonus grants +27 to your primary stat, and the three piece set bonus grants a two level bonus to one of your skills.
Also, starting with the most recent patch, you can obtain scout medallions from fallen enemies and turn them in for set pieces. The boots and gloves cost 50 and the chest costs 125.