Easy difficulty:
Lose twice on Normal.

Hard difficulty:
You must beat the game on Normal.

Critical (Issen) mode:
You must beat the game on Hard.

Oni Hard mode:
You must beat the game in Critical (Issen) mode.

Genma Puzzle game:
You must solve all puzzles in the game.

Secret Training mode game:
Finish the game and beat it on normal training mode.

Oni Shooting game:
Beat the game on Normal.

Ultimate mode:
Beat the Oni Shooting game.

Adventures of Heihachi mode:
With any difficulty beat the game.

Original Onimusha swords:
While beating the game acquire all 3 items from Phantom Realms. Then start a new game and play with all 3 swords from the original.

Bishamon sword:
Pick it up in the Phantom Realm in Honnouji or beat the game with Onimusha rank.

Ako's Black Haori:
Beat Issen/Oni training.

Michelle's alternate costume:
Beat the game on Normal.

Michelle's second alternate costume:
Play the game with a memory card inserted containing a saved game from Onimusha Blade Warriors or beat the game with Onimusha Rank.

Jacques' alternate costume:
With Jackques beat the game on normal or you can beat teh game with Samanusuke's alternate costume.

Samanosuke's alternate costume:
With Samanosuke beat the game on normal and get a high score. Or you can beat the game with Jacque's alternate costume.

Panda costume:
Play the game with a memory card inserted conataining a saved game from Onimusha Blade Warriors or Beat Secret Training mode.

Alternate ending:
Max out all seven Haori of Ako.