Die three times in the normal game, then you can access this by dying or the options menu. The enemies take less effort to kill, and you start out with much more healing items
Beat the game on normal. Enemies inflict more damage, are faster, and take more time to kill. Your healing items have less of an effect.
Clear the Team Oni mini game. Nothing can be killed except with an Issen strike in this mode. This even includes bosses.
Clear the game on Hard. You start the game with the following items.
Tanegashima with unlimited bullets
Tri-Barrel gun with unlimited burst bullets
Bow with unlimited arrows and fire arrows
Rekka-Ken fire sword
All of your armor at level 3
30 Perfect Medicine
Man in Black minigame:
Clear the game once on any mode. You play as Jubei in a black suit, and you run around hitting enemies with a sponge stick, collecting tapes. Once you finish this, you can view scenes from the game with Jubei in the black suit.
Clear the game once on any mode. You go through 21 levels of battles with any character that you have played as so far. Your health slowly drains but there are pots in the room that give off yellow souls to heal you with. Good luck and try to get the high score
Puzzle Phantom Realm:
Get all 18 artworks from the game to unlock this mind bending mini-game which has you solving 36 trick treasure boxes to get the remaining artworks.
Onimusha 3 Trailer - Clear the game once to view this 25 second long preview of Onimusha 3.
Jubeis extra costume:
Get an Onimusha rank on your final score.
Oyu's extra costume:
Clear all 100 chapters in the scenario route. I've recieved a TON of e-mails about this, but you only need THREE clears to do this.