Unlock Oni Spirits:
If you collect all 20 Flourites and finish the game, the mini-game Oni Spirits will be unlocked. For a full description of where to find each Flourite, read the Flourites section of this guide.

Unlock Onimusha 2 Trailer:
Finish the game on any difficulty to unlock a sneak preview of Onimusha: Warlords 2. When you go back to the main menu after saving, go to Special Feature to view the preview footage.

Alternate Costumes:
If you want your character to look a little different, simply complete the game one time and then start a new game after saving. You will see a "Normal/Special" option. If you select the Special option you will appear in a special panda suit.

Onimusha 2:
After you complete the game for the first time, it will prompt you to save. Do so, and then start a new game. View the "Special Report" to see a small preview of Onimusha 2, which apparently takes place about 10 years after the events in this game.

Ultimate mode:
You have to first get all the flouries in the game and then beat the oni mini game. Once you have done that you will have the ultimate mode with lots of goodies.