Oda Nobunaga was born on June 23, 1534 in Nagoya, Owari Province, and was the second son of Oda Nobuhide, the head of the powerful Oda clan and a deputy shugo. Nobunaga was given the childhood name of Kippōshi (吉法師), and through his childhood and early teenage years became well known for his bizarre behavior, receiving the name of Owari no Ōutsuke (尾張の大うつけ, The Fool of Owari). Nobunaga was a clear speaker with a strong presence about him, and known to run around with other youths from the area, without any regard to his own rank in society.

Nobuhide made peace with Saitō Dōsan by arranging a political marriage between his son and heir, Oda Nobunaga, and Saitō Dōsan daughter, Nōhime. Dōsan became the father-in-law of Oda Nobunaga.

In 1551, Oda Nobuhide died unexpectedly. It has been said that Nobunaga acted outrageously during his funeral, throwing ceremonial incense at the altar. Although Nobunaga was Nobuhide's legitimate heir, a succession crisis occurred when some of the Oda clan were divided against him. Nobunaga, collecting about a thousand men, suppressed members of his family who were hostile to his rule and their allies.

After Oda Nobuhide died in 1551, Nobuhide's son Nobunaga was initially unable to assume control of the entire clan. Nobutomo challenged Nobunaga for control of Owari in the name of Owari's shugo, Shiba Yoshimune, technically his superior but in reality his puppet. After Yoshimune revealed to Nobunaga an assassination plot in 1554, Nobutomo had Yoshimune put to death. The next Year, Nobunaga took Kiyosu Castle and captured Nobutomo, forcing him to commit suicide not long after.

Nobunaga sent an army to Mino Province to aid his father-in-law, Saitō Dōsan, after Dōsan's son, Saitō Yoshitatsu, turned against him, but they did not reach the battle in time to offer any help. Dōsan was killed in the Battle of Nagara-gawa, and Yoshitatsu became the new master of Mino.

Nobunaga's main rival as head of the Oda clan was his younger brother, Oda Nobuyuki. In 1555, Nobunaga defeated Nobuyuki at the Battle of Ino, though Nobuyuki survived and began plotting a second rebellion.

Nobuyuki was defeated by Nobunaga's retainer Ikeda Nobuteru. Nobuyuki conspired against his brother Nobunaga with the Hayashi clan (Owari), which Nobunaga viewed as treason. When Nobunaga was informed of this by Shibata Katsuie, he faked illness to get close to Nobuyuki and assassinated him in Kiyosu Castle.

Suzuki Shigeteru, lord of Terabe Castle, defected from the Imagawa in favor of an alliance with Oda Nobunaga. The Imagawa responded by sending an army under the command of Matsudaira Motoyasu, a young vassal of Imagawa Yoshimoto. Terabe Castle was the first of a series of battles waged against the Oda clan.

Nobunaga had captured and obliterated the fortress of Iwakura, eliminated all opposition within the Oda clan and established his uncontested rule in Owari Province.

Imagawa Yoshimoto was a long-time opponent of Nobunaga's father, and had sought to expand his domain into Oda territory in Owari. In 1560, Imagawa Yoshimoto gathered an army of 25,000 men, and started his march toward the capital city of Kyoto, with the pretext of aiding the frail Ashikaga Shogunate. The Matsudaira clan also joined Yoshimoto's forces. The Imagawa forces quickly overran the border fortresses of Washizu, Matsudaira forces led by Matsudaira Motoyasu took Marune Fortress. Against this, the Oda clan could rally an army of only 2,000 to 3,000 men. Some of his advisors suggested "to stand a siege at Kiyosu" but Nobunaga refused, stating that "only a strong offensive policy could make up for the superior numbers of the enemy", and calmly ordered a counterattack against Yoshimoto.

In June 1560, Nobunaga's scouts reported that Yoshimoto was resting at the narrow gorge of Dengaku-hazama, ideal for a surprise attack, and that the Imagawa army was celebrating their victories of Washizu and Marune fortress. Nobunaga ordered his men to set up an array of flags and dummy troops made of straw and spare helmets around the Zensho-ji, giving the impression of a large host, while the real Oda army hurried round in a rapid march to get behind Yoshimoto's camp. Nobunaga deployed his troops at Kamagatani. When the storm ceased, they charged down upon the enemy. At first, Yoshimoto thought a brawl had broken out among his men, but then he realized that it was an attack when two of Nobunaga's samurais, Mōri Shinsuke and Hattori Koheita, charged up at him. One aimed a spear at him, which Yoshimoto deflected with his sword, but the second swung his blade and decapitated him. With his victory in this battle, Oda Nobunaga gained greatly in prestige, and many samurai and warlords pledged fealty to him.

In 1567, Inaba Ittetsu along with Andō Michitari and Ujiie Bokuzen, agreed to join the forces of Oda Nobunaga. Eventually, they mounted a victorious final attack at the Siege of Inabayama Castle. After taking possession of the castle, Nobunaga changed the name of both Inabayama Castle and the surrounding town to Gifu. Nobunaga revealed his ambition to conquer the whole of Japan. In about two weeks' time Nobunaga had entered the sprawling Mino Province, raised an army, and conquered the ruling clan in their mountaintop castle. Following the battle the Mino Triumvirate, awed by the speed and skill of Nobunaga's conquest, permanently allied themselves to Nobunaga.

In 1568, Ashikaga Yoshiaki and Akechi Mitsuhide, as Yoshiaki's bodyguard, went to Gifu to ask Nobunaga to start a campaign toward Kyoto. Yoshiaki was the brother of the murdered 13th shōgun of the Ashikaga Shogunate, Yoshiteru, and wanted revenge against the killers who had already set up a puppet shōgun, Ashikaga Yoshihide. Nobunaga agreed to install Yoshiaki as the new shōgun, and grasping the opportunity to enter Kyoto, started his campaign.

Nobunaga entered Kyoto, drove out the Miyoshi clan, who fled to Settsu, and installed Yoshiaki as the 15th shōgun of the Ashikaga Shogunate. However, Nobunaga refused the title of Shōgun's deputy (Kanrei), or any appointment from Yoshiaki. As their relationship grew difficult, Yoshiaki secretly started an anti-Nobunaga alliance, conspiring with other daimyos to get rid of Nobunaga, though Nobunaga had a great respect with the Emperor Ōgimachi.

After installing Yoshiaki as Shogun, Nobunaga had evidently pressed Yoshiaki to request all the local Daimyô to come to Kyôto and attend a certain banquet. Asakura Yoshikage, head of the Asakura clan was the regent of Ashikaga Yoshiaki, refused, an act Nobunaga declared disloyal to both the shogun and the emperor. With this pretext well in hand, Nobunaga raised an army and marched on Echizen.

In early 1570, Nobunaga launched a campaign into the Asakura clan's domain and besieged Kanagasaki Castle. Azai Nagamasa, to whom Nobunaga's sister Oichi was married, broke the alliance with the Oda clan to honor the Azai-Asakura alliance. With the help of the Rokkaku clan and the Ikkō-ikki, the anti-Nobunaga alliance sprang into full force, taking a heavy toll on the Oda clan. Nobunaga found himself facing both the Asakura and Azai forces and when defeat looked certain, Nobunaga decided to retreat from Kanagasaki, which went successfully.

In July 1570, the Oda-Tokugawa allies marched on Yokoyama and Odani Castles, and the combined Azai-Asakura force marched out to confront Nobunaga. Tokugawa Ieyasu joined his forces with Nobunaga, with the Oda and Azai clashing on the right while Tokugawa and Asakura grappled on the left. The battle turned into a melee fought in the middle of the shallow Ane River. For a time, Nobunaga's forces fought the Azai upstream, while the Tokugawa warriors fought the Asakura downstream. After the Tokugawa forces finished off the Asakura, they turned and hit the Azai right flank. The troops of the Mino Triumvirate, who had been held in reserve, then came forward and hit the Azai left flank. Soon both the Oda and Tokugawa forces defeated the combined forces of the Asakura and Azai clans.

The Siege of Mount Hiei (比叡山の戦い) was a battle of the Sengoku period of Japan fought between Oda Nobunaga and the sōhei (warrior monks) of the monasteries of Mount Hiei near Kyoto on 29 September 1571. Nobunaga and Akechi Mitsuhide led 30,000 men to Mount Hiei, destroying towns and temples on the mountain or near its base, and killing their residents without exemption. Nobunaga killed an estimated 20,000 people and around 300 buildings were burned to the ground, ending the great power of the Mount Hiei warrior monks.

In 1573, at the Siege of Odani Castle and the Siege of Ichijōdani Castle, Nobunaga successfully destroyed the Asakura and Azai clans by driving them both to the point that the clan leaders committed suicide.

In 1575, Takeda Katsuyori, son of Takeda Shingen, attacked Nagashino Castle when Okudaira Sadamasa rejoined the Tokugawa and his original plot with Oga Yashiro to take Okazaki Castle, the capital of Mikawa, was discovered. Ieyasu appealed to Nobunaga for help and Nobunaga personally lead an army of about 30,000 men. The combined force of 38,000 men under Nobunaga and Tokugawa Ieyasu defeated and devastated the Takeda clan with the strategic use of arquebuses at the decisive battle in Nagashino. Nobunaga compensated for the arquebus' slow reloading time by organizing the arquebusiers in three rows, firing in rotation. Takeda Katsuyori also wrongly assumed that rain had ruined the gunpowder of Nobunaga's forces.

Several times in Japanese history, the new ruler sought to ensure his position by calling a sword hunt (刀狩, katanagari). Armies would scour the entire country, confiscating the weapons of the enemies of the new regime.

Most men wore swords, from Heian period until Sengoku period in Japan. Oda Nobunaga sought an end to this practice, and ordered the seizure of swords and a variety of other weapons from civilians, in particular the Ikkō-ikki peasant-monk leagues which sought to overthrow samurai rule.

Tenshō Iga War were two invasions of Iga province by the Oda clan during the Sengoku period. The province was conquered by Oda Nobunaga in 1581 after an unsuccessful attempt in 1579 by his son Oda Nobukatsu. The names of the wars are derived from the Tenshō era name (1573–92) in which they occurred. Oda Nobunaga himself toured the conquered province in early November 1581, and then withdrew his troops, placing control in Nobukatsu's hands.

Akechi Mitsuhide, stationed in the Chūgoku region, decided to assassinate Nobunaga for unknown reasons, and the cause of his betrayal is controversial. Mitsuhide, aware that Nobunaga was nearby and unprotected for his tea ceremony, saw an opportunity to act. The Akechi army had the Honnō-ji temple surrounded in a coup d'état. Nobunaga and his servants and bodyguards resisted, but they realized it was futile against the overwhelming numbers of Akechi troops. Nobunaga then, with the help of his young page, Mori Ranmaru, committed seppuku. Reportedly, Nobunaga's last words were "Ran, don't let them come in ..." to Ranmaru, who then set the temple on fire as Nobunaga requested so that no one would be able to get his head. After capturing Honnō-ji, Mitsuhide attacked Nobunaga's eldest son and heir, Oda Nobutada, who was staying at the nearby Nijō Palace. Nobutada also committed seppuku.

Later, Nobunaga retainer Toyotomi Hideyoshi, subsequently abandoned his campaign against the Mōri clan to pursue Mitsuhide to avenge his beloved lord. Hideyoshi's intercepted one of Mitsuhide's messengers trying to deliver a letter to the Mōri requesting to form an alliance against the Oda after informing them of Nobunaga's death. Hideyoshi managed to pacify the Mōri by demanding the suicide of Shimizu Muneharu in exchange for ending his siege of Takamatsu Castle, which the Mōri accepted. Mitsuhide failed to establish his position after Nobunaga's death and Oda forces under Hideyoshi defeated his army at the Battle of Yamazaki in July 1582, but Mitsuhide was murdered by bandits while fleeing after the battle. Hideyoshi continued and completed Nobunaga's conquest of Japan within the following decade.