The battle of Okehazama changed Japan drastically in my opinion. Okehazama was the shot heard around Japan and Nobunaga was now going to be the new boss in town, like it or not. Also, Okehazama freed another Sengoku legend, Matsudaira Motoyasu (Tokugawa Ieyasu).
It seems that the Oda and Tokugawa were still fighting after Okehazama. However, Ieyasu was not in a position where he could fight the Oda on a constant day in or day out basis. It was a good thing that Ieyasu took his uncle's advice seriously. It was Mizuno Nobumoto of Kariya who was on good terms with the Oda convinced Ieyasu to have an alliance with Nobunaga. Without it the alliance, Ieyasu's chances to unify Mikawa would have become more difficult. The same goes for Nobunaga, he would have a harder time taking over Mino.
However, there were people in Ieyasu's camp who disagreed with the alliance. Sakai Tadanao, who served the Matsudaira since the fifteenth century strongly disapproved and thought it was a big mistake from the beginning. Carol Richmond Tsang pointed out in her book War and Faith, it was unclear how Sakai expected Ieyasu to maintain ties with the Imagawa (p. 207).
For Nobunaga, the next twenty-two years he would change Japan's military, economics, and culture forever. For example, guns, gold, intelligence, and public works projects. He paved the way to unite Japan under his slogan "Tenka Fubu" and Okehazama started the unification process.
Tenka no tame!