The foundation of current Japanese society was primarily formed during Edo periodo, (1603-1867). And that administration was completed by three generals: Oda Nobunaga, Toyotomi Hideyoshi, and Tokugawa Ieyasu, after the warring states period. Osaka Castle was profoundly involved those three generals' straggles for unification of Japan.
The place where Osaka Castle stands was previously a Buddhist temple called Ishiyama Hongan-ji, one of the important bases of Jodo Shinshu, a then emergent denomination of Buddhism.
Oda Nobunaga, the man of supreme power in those days, considered its prosperity to be a threat against his administration, and therefor oppressed and seized it.
Oda Nobunaga died just before completing the unification of Japan, and then Toyotmi Hideyoshi, a faithful subordinate of Nobunaga, completed it, taking over his power base.
The construction of Osaka Castle started in 1853 by Toyotomi Hideyoshi at the vacant lot of Ishiyama Honganji; he had been intending to make it a symble of his authority. While Hideyoshi professed himself to be a legitimate successor of Nobunaga, he wanted to build a castle which was superior to Azuchi Castle, the principal castle of Nobunaga.
After the death of Hideyoshi, the members of his administration were split into two factions, which ended up in a war called the Battle of Sekigahara; it was a battle between Eastern Army and Western Army. The result of that war was a victory of Eastern Army led by Tokugawa Ieyasu while Western Army led by Ishida Mitsunari, a more faithful subordinate of Hideyoshi than Ieyasu, was defeated.
Consequently, Ieyasu effectively grasped the power and established his government in Edo (Tokyo), while Toyotomi family fell into one of the local rulers based in Osaka Castle.
Toyotomi family, led by Toyotomi Hideyori and Yodo, a son of Hideyoshi and his mother, was eventually destroyed at Osaka Castle by Ieyasu's troops through two battles, which were so-called "Winter and Summer Campaign of siege of Osaka".
After that, Ieyasu consolidated his administration and Edo Period continued until 1867. The management of Osaka castle was succeeded to Ieyasu's relations and the senior executives of his administration through Edo Period, undergoing several fires and repair works.
Edo Period came to an end with the Meiji Restoration, which effectively was a revolution by local powers who were getting more influences as the Administration of Tokugawa had weakened.
The prototype of today’s Japanese government was established as a result of the Meiji Restoration, with Osaka Castle inevitably seized by the new government. At present, Osaka Castle has been managed by Osaka City, although its registered owner has yet to be transferred from previous Japanese army; which was recently revealed by Mainichi Shinbun Co., Ltd., one of the major newspaper and publishing company of Japan, and became a controversial news.