Hm, if we think that Oathbringer is a legendary sword, that implies that there must have been a legendary swordsman who once wielded it. And since shardblades are so large, I think we have a good idea of who that was. He even has a unique shardplate variation (berserker armor, for those unfamiliar with the manga):
(Please do this, Brandon, I NEED something to distract me from the travesty of the 2016 Berserk Anime)