Though this is personal experience, it takes approximately 15 minutes to make a complete, but not fully upgraded, Oscar cosplay.
Start as a Knight, complete the tutorial. Then get 14 Faith and run down to the Valley of Drakes to get the Astora's Straight Sword and Dragon Crest Shield (it helps later). Run to the Darkroot Garden (by going to the elevator, running past the Black Knight, and going up.) Run from the little puddle to the fog gate, then make a dash for the Elite Knight set. After you get it, run all the way to the titanite Demon in the Parish, around it, and then go to the Church to activate the elevator and then hop off do where you would to go the Asylum. Enter the Asylum, kill the Hollows, and head to Oscar's spot. Kill him, and you get the Crest Shield. If you have a Homeward Bone, use it. Otherwise, run all the way back to the nest, and then hit the Firelink bonfire.
Boom. Oscar cosplay in 15-20 minutes.