Is okami a girl or boy?

Yes, she is a girl. Her name isn't actually Okami. The Word, "Okami," is actually a play on Word that means both, "great god," and, "wolf" in Japanese.

Her name is Amaterasu, sometimes shortened to, "Ammy in the game." In the game she is called, "girl" by several travelers.

More proof that she is a female is that her Japanese god counterpart is a female, and that in the upcoming sequel, Okamiden, you play as the child she birthed.