VGFacts

When you meet Tama (The fireworks guy) he shows you a fireworks show titled "The Midnight Wonder Boy." a reference to Alastor from Viewtiful Joe, as his chapter in the game is called "The Midnight Thunderboy."

(at 12:05)

http://okami.wikia.com/wiki/Clover#Trivia

The Clovers in Ōkami are believed to be a reference to Clover Studio which developed the game. This theory is further proved because the tune they make when dug up is very similar to the tune played during Clover's opening title.
Due to Clover Studio's ending, this may also be the reason why clovers do not appear in Ōkamiden, and are instead replaced by Dandelions.

http://okami.neoseeker.com/wiki/Dandelions