Complete normal training mode, and complete the game.
Shooting Mini Game
Complete the game on Normal difficulty.
Michelle's Costume
Complete the game with Onimusha ranking.
Panda Costume
Complete Secret Training mode.
Easy Mode
Get "Game Over" twice.
Hard Mode
Complete the game on normal mode.
Issen Mode
Complete game on hard mode.
Oni Mode
Complete Issen mode.
Ultimate Mode
Complete the shooting mini game.
Unlock the Three Swords of Onimusha
Complete all of the phantim realms, finish the game, and start new game.
Heihachi Buraiden Mini Game
Finish the game.
Quick Health
In the Zoo, Stand behind the panda cutout and equip the white vest on Ako and watch your health go up quickly.
Unlimited Souls
To acquire an unlimited amount of souls, after obtaining the Oni Army Orb, get to second area. Kill off initial wave of foes, and find something to hold down the soul absorption button. Your Oni army will continue to defeat the spawning Genma, and you will receive all their souls. This will take some time before acquiring a large amount.
Panda Suit Photo Opportunity
In the Bolougne Zoo, just outside of the building with the air-sealed door, there is a wooden photo shoot prop. Stand behind it and the camera shows your character in the panda suit from Onimusha Warlords, with Ako as the baby panda.
Samanosuke’s Warlords Weapons
To unlock the three weapons that Samanosuke wielded in Onimusha: Warlords obtain the Fire Seal, the Thunder Seal and the Wind Seal and complete the game. The next time the game is played through the three weapons reappear in the alleyway where Samanosuke materializes in the present time period.
Play as Gargant
Gargant can become a playable character in Onimusha: Blade Warriors. Have a saved game with a clear status on a memory card in slot 1 when booting up Blade Warriors.
Mini Games
Oni Target Practice: Complete the game on a normal setting to unlock oni target practice. Character must use guns or arrows to pick off Genma eggs that go by on tracks.
Adventures of Heihachi: Complete the game on a normal setting to unlock this short storyline featuring Heihachi.
Oni Training: Complete every training session and beat the game to unlock this bonus training session.
Genma Puzzle Space: Solve every Puzzle Treasure Box in the game and complete the game to unlock this mini-game.
Ranking Mode
Complete the game on normal difficulty. Two ranking screens will be available. The first one displays the individual scores for each character while the second displays an overall letter grade and any earned titles.
I know it may not sound real but it is true! There is a 4th Dark Realm.
The 4th Dark Realm is inside the Oni Mansion. Once you are inside the mansion reach the end of the maze.(Beside the protective mirror.) Hit with any of your weapons the pink screen. Enter the small passageway where on your left is a hole and on your right is a bundle of boxes.
DO NOT ENTER THE HOLE!
Keep pressing the X button on the boxes. After whenabout 2-5 seconds the man, wrapped in the green material who hangs from the ceiling, shall appear. If you press the X button on him he shall ask you if you would like to go to the Dark Realm.
Note:
1. I have only tried this on Hard Mode.
2. The reward should help you in the next battle!
3. Obviously the Oni Mansion is only available when playing as Jacques.
Unlock alternate ending sequence
Increase all seven Haori of Ako to the maximum. This requires all Kodama to be found.
Unlock Bishamon sword
Successfully complete the game with the "Onimusha" rank. Alternately, get the Bishamon sword in the Phantom Realm in Honnouji when Sam goes to the past.
Unlock Oni Musou
Have Jacque go through the Phantom Realm in Honnouji.