Omochao is also a Chao! (except a robotic one...).
He will act as your guide throughout the game and can be found at various locations throughout most levels.
You can even grab him and throw him at enemies! (although he won't be too happy)
Omochao also gives you a tutorial that will show you the basics of Sonic Adventure 2, Chao, and much more.
You also race him in one of the Chao Races! (See Chao Races 1 for more information).
The first appearance of Omochao was in Phantasy Star Online for the Dreamcast. In this came you could obtain items called "Mag's" to give your character stat boosts.
One such Mag was a Chao, and another was a Robochao (which is actually Omochao).
So... Can you get Omochao in your Chao Garden normally? Sadly, no (or is that a good thing?).
The only way to obtain an Omochao in your garden is by using a cheat device or something similar. The codes will be in the codes/hacking section.